diff --git a/src/context.c b/src/context.c
index 3da19864..455beeab 100644
--- a/src/context.c
+++ b/src/context.c
@@ -27,6 +27,7 @@
 
 #include "internal.h"
 
+#include <assert.h>
 #include <stdio.h>
 #include <string.h>
 #include <limits.h>
@@ -566,6 +567,7 @@ GLFWAPI GLFWwindow* glfwGetCurrentContext(void)
 GLFWAPI void glfwSwapBuffers(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -595,6 +597,8 @@ GLFWAPI int glfwExtensionSupported(const char* extension)
 {
     _GLFWwindow* window;
 
+    assert(extension);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(GLFW_FALSE);
 
     window = _glfwPlatformGetCurrentContext();
@@ -657,6 +661,8 @@ GLFWAPI int glfwExtensionSupported(const char* extension)
 
 GLFWAPI GLFWglproc glfwGetProcAddress(const char* procname)
 {
+    assert(procname);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     if (!_glfwPlatformGetCurrentContext())
diff --git a/src/input.c b/src/input.c
index db33ff72..01184c32 100644
--- a/src/input.c
+++ b/src/input.c
@@ -27,6 +27,7 @@
 
 #include "internal.h"
 
+#include <assert.h>
 #include <stdlib.h>
 
 // Internal key state used for sticky keys
@@ -239,6 +240,7 @@ GLFWbool _glfwIsPrintable(int key)
 GLFWAPI int glfwGetInputMode(GLFWwindow* handle, int mode)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT_OR_RETURN(0);
 
@@ -259,6 +261,7 @@ GLFWAPI int glfwGetInputMode(GLFWwindow* handle, int mode)
 GLFWAPI void glfwSetInputMode(GLFWwindow* handle, int mode, int value)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -288,6 +291,7 @@ GLFWAPI const char* glfwGetKeyName(int key, int scancode)
 GLFWAPI int glfwGetKey(GLFWwindow* handle, int key)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT_OR_RETURN(GLFW_RELEASE);
 
@@ -310,6 +314,7 @@ GLFWAPI int glfwGetKey(GLFWwindow* handle, int key)
 GLFWAPI int glfwGetMouseButton(GLFWwindow* handle, int button)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT_OR_RETURN(GLFW_RELEASE);
 
@@ -333,6 +338,7 @@ GLFWAPI int glfwGetMouseButton(GLFWwindow* handle, int button)
 GLFWAPI void glfwGetCursorPos(GLFWwindow* handle, double* xpos, double* ypos)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     if (xpos)
         *xpos = 0;
@@ -355,6 +361,7 @@ GLFWAPI void glfwGetCursorPos(GLFWwindow* handle, double* xpos, double* ypos)
 GLFWAPI void glfwSetCursorPos(GLFWwindow* handle, double xpos, double ypos)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -378,6 +385,8 @@ GLFWAPI GLFWcursor* glfwCreateCursor(const GLFWimage* image, int xhot, int yhot)
 {
     _GLFWcursor* cursor;
 
+    assert(image);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     cursor = calloc(1, sizeof(_GLFWcursor));
@@ -462,6 +471,7 @@ GLFWAPI void glfwSetCursor(GLFWwindow* windowHandle, GLFWcursor* cursorHandle)
 {
     _GLFWwindow* window = (_GLFWwindow*) windowHandle;
     _GLFWcursor* cursor = (_GLFWcursor*) cursorHandle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -473,6 +483,8 @@ GLFWAPI void glfwSetCursor(GLFWwindow* windowHandle, GLFWcursor* cursorHandle)
 GLFWAPI GLFWkeyfun glfwSetKeyCallback(GLFWwindow* handle, GLFWkeyfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.key, cbfun);
     return cbfun;
@@ -481,6 +493,8 @@ GLFWAPI GLFWkeyfun glfwSetKeyCallback(GLFWwindow* handle, GLFWkeyfun cbfun)
 GLFWAPI GLFWcharfun glfwSetCharCallback(GLFWwindow* handle, GLFWcharfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.character, cbfun);
     return cbfun;
@@ -489,6 +503,8 @@ GLFWAPI GLFWcharfun glfwSetCharCallback(GLFWwindow* handle, GLFWcharfun cbfun)
 GLFWAPI GLFWcharmodsfun glfwSetCharModsCallback(GLFWwindow* handle, GLFWcharmodsfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.charmods, cbfun);
     return cbfun;
@@ -498,6 +514,8 @@ GLFWAPI GLFWmousebuttonfun glfwSetMouseButtonCallback(GLFWwindow* handle,
                                                       GLFWmousebuttonfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.mouseButton, cbfun);
     return cbfun;
@@ -507,6 +525,8 @@ GLFWAPI GLFWcursorposfun glfwSetCursorPosCallback(GLFWwindow* handle,
                                                   GLFWcursorposfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.cursorPos, cbfun);
     return cbfun;
@@ -516,6 +536,8 @@ GLFWAPI GLFWcursorenterfun glfwSetCursorEnterCallback(GLFWwindow* handle,
                                                       GLFWcursorenterfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.cursorEnter, cbfun);
     return cbfun;
@@ -525,6 +547,8 @@ GLFWAPI GLFWscrollfun glfwSetScrollCallback(GLFWwindow* handle,
                                             GLFWscrollfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.scroll, cbfun);
     return cbfun;
@@ -533,6 +557,8 @@ GLFWAPI GLFWscrollfun glfwSetScrollCallback(GLFWwindow* handle,
 GLFWAPI GLFWdropfun glfwSetDropCallback(GLFWwindow* handle, GLFWdropfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.drop, cbfun);
     return cbfun;
@@ -553,6 +579,7 @@ GLFWAPI int glfwJoystickPresent(int joy)
 
 GLFWAPI const float* glfwGetJoystickAxes(int joy, int* count)
 {
+    assert(count);
     *count = 0;
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
@@ -568,6 +595,7 @@ GLFWAPI const float* glfwGetJoystickAxes(int joy, int* count)
 
 GLFWAPI const unsigned char* glfwGetJoystickButtons(int joy, int* count)
 {
+    assert(count);
     *count = 0;
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
@@ -597,6 +625,10 @@ GLFWAPI const char* glfwGetJoystickName(int joy)
 GLFWAPI void glfwSetClipboardString(GLFWwindow* handle, const char* string)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
+    assert(string);
+
     _GLFW_REQUIRE_INIT();
     _glfwPlatformSetClipboardString(window, string);
 }
@@ -604,6 +636,8 @@ GLFWAPI void glfwSetClipboardString(GLFWwindow* handle, const char* string)
 GLFWAPI const char* glfwGetClipboardString(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     return _glfwPlatformGetClipboardString(window);
 }
diff --git a/src/monitor.c b/src/monitor.c
index f9ab7b0d..1aebf0ba 100644
--- a/src/monitor.c
+++ b/src/monitor.c
@@ -27,6 +27,7 @@
 
 #include "internal.h"
 
+#include <assert.h>
 #include <math.h>
 #include <float.h>
 #include <string.h>
@@ -293,6 +294,7 @@ void _glfwSplitBPP(int bpp, int* red, int* green, int* blue)
 
 GLFWAPI GLFWmonitor** glfwGetMonitors(int* count)
 {
+    assert(count);
     *count = 0;
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
@@ -314,6 +316,7 @@ GLFWAPI GLFWmonitor* glfwGetPrimaryMonitor(void)
 GLFWAPI void glfwGetMonitorPos(GLFWmonitor* handle, int* xpos, int* ypos)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor);
 
     if (xpos)
         *xpos = 0;
@@ -328,6 +331,7 @@ GLFWAPI void glfwGetMonitorPos(GLFWmonitor* handle, int* xpos, int* ypos)
 GLFWAPI void glfwGetMonitorPhysicalSize(GLFWmonitor* handle, int* widthMM, int* heightMM)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor);
 
     if (widthMM)
         *widthMM = 0;
@@ -345,6 +349,8 @@ GLFWAPI void glfwGetMonitorPhysicalSize(GLFWmonitor* handle, int* widthMM, int*
 GLFWAPI const char* glfwGetMonitorName(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     return monitor->name;
 }
@@ -359,7 +365,9 @@ GLFWAPI GLFWmonitorfun glfwSetMonitorCallback(GLFWmonitorfun cbfun)
 GLFWAPI const GLFWvidmode* glfwGetVideoModes(GLFWmonitor* handle, int* count)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor);
 
+    assert(count);
     *count = 0;
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
@@ -374,6 +382,7 @@ GLFWAPI const GLFWvidmode* glfwGetVideoModes(GLFWmonitor* handle, int* count)
 GLFWAPI const GLFWvidmode* glfwGetVideoMode(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor);
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
@@ -422,6 +431,7 @@ GLFWAPI void glfwSetGamma(GLFWmonitor* handle, float gamma)
 GLFWAPI const GLFWgammaramp* glfwGetGammaRamp(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor);
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
@@ -434,6 +444,9 @@ GLFWAPI const GLFWgammaramp* glfwGetGammaRamp(GLFWmonitor* handle)
 GLFWAPI void glfwSetGammaRamp(GLFWmonitor* handle, const GLFWgammaramp* ramp)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor);
+
+    assert(ramp);
 
     _GLFW_REQUIRE_INIT();
 
diff --git a/src/window.c b/src/window.c
index eff61be2..f0063e35 100644
--- a/src/window.c
+++ b/src/window.c
@@ -28,6 +28,7 @@
 
 #include "internal.h"
 
+#include <assert.h>
 #include <string.h>
 #include <stdlib.h>
 
@@ -124,6 +125,8 @@ GLFWAPI GLFWwindow* glfwCreateWindow(int width, int height,
     _GLFWwindow* window;
     _GLFWwindow* previous;
 
+    assert(title);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     if (width <= 0 || height <= 0)
@@ -415,6 +418,8 @@ GLFWAPI void glfwDestroyWindow(GLFWwindow* handle)
 GLFWAPI int glfwWindowShouldClose(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(0);
     return window->closed;
 }
@@ -422,6 +427,8 @@ GLFWAPI int glfwWindowShouldClose(GLFWwindow* handle)
 GLFWAPI void glfwSetWindowShouldClose(GLFWwindow* handle, int value)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT();
     window->closed = value;
 }
@@ -429,6 +436,10 @@ GLFWAPI void glfwSetWindowShouldClose(GLFWwindow* handle, int value)
 GLFWAPI void glfwSetWindowTitle(GLFWwindow* handle, const char* title)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
+    assert(title);
+
     _GLFW_REQUIRE_INIT();
     _glfwPlatformSetWindowTitle(window, title);
 }
@@ -436,6 +447,7 @@ GLFWAPI void glfwSetWindowTitle(GLFWwindow* handle, const char* title)
 GLFWAPI void glfwGetWindowPos(GLFWwindow* handle, int* xpos, int* ypos)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     if (xpos)
         *xpos = 0;
@@ -449,6 +461,7 @@ GLFWAPI void glfwGetWindowPos(GLFWwindow* handle, int* xpos, int* ypos)
 GLFWAPI void glfwSetWindowPos(GLFWwindow* handle, int xpos, int ypos)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -465,6 +478,7 @@ GLFWAPI void glfwSetWindowPos(GLFWwindow* handle, int xpos, int ypos)
 GLFWAPI void glfwGetWindowSize(GLFWwindow* handle, int* width, int* height)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     if (width)
         *width = 0;
@@ -478,6 +492,7 @@ GLFWAPI void glfwGetWindowSize(GLFWwindow* handle, int* width, int* height)
 GLFWAPI void glfwSetWindowSize(GLFWwindow* handle, int width, int height)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -495,6 +510,7 @@ GLFWAPI void glfwSetWindowSizeLimits(GLFWwindow* handle,
                                      int maxwidth, int maxheight)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -509,6 +525,7 @@ GLFWAPI void glfwSetWindowSizeLimits(GLFWwindow* handle,
 GLFWAPI void glfwSetWindowAspectRatio(GLFWwindow* handle, int numer, int denom)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -527,6 +544,7 @@ GLFWAPI void glfwSetWindowAspectRatio(GLFWwindow* handle, int numer, int denom)
 GLFWAPI void glfwGetFramebufferSize(GLFWwindow* handle, int* width, int* height)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     if (width)
         *width = 0;
@@ -542,6 +560,7 @@ GLFWAPI void glfwGetWindowFrameSize(GLFWwindow* handle,
                                     int* right, int* bottom)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     if (left)
         *left = 0;
@@ -559,6 +578,8 @@ GLFWAPI void glfwGetWindowFrameSize(GLFWwindow* handle,
 GLFWAPI void glfwIconifyWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT();
     _glfwPlatformIconifyWindow(window);
 }
@@ -566,6 +587,8 @@ GLFWAPI void glfwIconifyWindow(GLFWwindow* handle)
 GLFWAPI void glfwRestoreWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT();
     _glfwPlatformRestoreWindow(window);
 }
@@ -573,6 +596,7 @@ GLFWAPI void glfwRestoreWindow(GLFWwindow* handle)
 GLFWAPI void glfwShowWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -585,6 +609,7 @@ GLFWAPI void glfwShowWindow(GLFWwindow* handle)
 GLFWAPI void glfwHideWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT();
 
@@ -597,6 +622,7 @@ GLFWAPI void glfwHideWindow(GLFWwindow* handle)
 GLFWAPI int glfwGetWindowAttrib(GLFWwindow* handle, int attrib)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
 
     _GLFW_REQUIRE_INIT_OR_RETURN(0);
 
@@ -643,6 +669,8 @@ GLFWAPI int glfwGetWindowAttrib(GLFWwindow* handle, int attrib)
 GLFWAPI GLFWmonitor* glfwGetWindowMonitor(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     return (GLFWmonitor*) window->monitor;
 }
@@ -650,6 +678,8 @@ GLFWAPI GLFWmonitor* glfwGetWindowMonitor(GLFWwindow* handle)
 GLFWAPI void glfwSetWindowUserPointer(GLFWwindow* handle, void* pointer)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT();
     window->userPointer = pointer;
 }
@@ -657,6 +687,8 @@ GLFWAPI void glfwSetWindowUserPointer(GLFWwindow* handle, void* pointer)
 GLFWAPI void* glfwGetWindowUserPointer(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     return window->userPointer;
 }
@@ -665,6 +697,8 @@ GLFWAPI GLFWwindowposfun glfwSetWindowPosCallback(GLFWwindow* handle,
                                                   GLFWwindowposfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.pos, cbfun);
     return cbfun;
@@ -674,6 +708,8 @@ GLFWAPI GLFWwindowsizefun glfwSetWindowSizeCallback(GLFWwindow* handle,
                                                     GLFWwindowsizefun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.size, cbfun);
     return cbfun;
@@ -683,6 +719,8 @@ GLFWAPI GLFWwindowclosefun glfwSetWindowCloseCallback(GLFWwindow* handle,
                                                       GLFWwindowclosefun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.close, cbfun);
     return cbfun;
@@ -692,6 +730,8 @@ GLFWAPI GLFWwindowrefreshfun glfwSetWindowRefreshCallback(GLFWwindow* handle,
                                                           GLFWwindowrefreshfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.refresh, cbfun);
     return cbfun;
@@ -701,6 +741,8 @@ GLFWAPI GLFWwindowfocusfun glfwSetWindowFocusCallback(GLFWwindow* handle,
                                                       GLFWwindowfocusfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.focus, cbfun);
     return cbfun;
@@ -710,6 +752,8 @@ GLFWAPI GLFWwindowiconifyfun glfwSetWindowIconifyCallback(GLFWwindow* handle,
                                                           GLFWwindowiconifyfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.iconify, cbfun);
     return cbfun;
@@ -719,6 +763,8 @@ GLFWAPI GLFWframebuffersizefun glfwSetFramebufferSizeCallback(GLFWwindow* handle
                                                               GLFWframebuffersizefun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     _GLFW_SWAP_POINTERS(window->callbacks.fbsize, cbfun);
     return cbfun;