diff --git a/src/input.c b/src/input.c
index 01da7d65..c2a7a86d 100644
--- a/src/input.c
+++ b/src/input.c
@@ -44,6 +44,13 @@
 #define _GLFW_JOYSTICK_BUTTON   2
 #define _GLFW_JOYSTICK_HATBIT   3
 
+#define GLFW_MOD_MASK (GLFW_MOD_SHIFT | \
+                       GLFW_MOD_CONTROL | \
+                       GLFW_MOD_ALT | \
+                       GLFW_MOD_SUPER | \
+                       GLFW_MOD_CAPS_LOCK | \
+                       GLFW_MOD_NUM_LOCK)
+
 // Initializes the platform joystick API if it has not been already
 //
 static GLFWbool initJoysticks(void)
@@ -266,6 +273,12 @@ static GLFWbool parseMapping(_GLFWmapping* mapping, const char* string)
 //
 void _glfwInputKey(_GLFWwindow* window, int key, int scancode, int action, int mods)
 {
+    assert(window != NULL);
+    assert(key >= 0 || key == GLFW_KEY_UNKNOWN);
+    assert(key <= GLFW_KEY_LAST);
+    assert(action == GLFW_PRESS || action == GLFW_RELEASE);
+    assert(mods == (mods & GLFW_MOD_MASK));
+
     if (key >= 0 && key <= GLFW_KEY_LAST)
     {
         GLFWbool repeated = GLFW_FALSE;
@@ -297,6 +310,10 @@ void _glfwInputKey(_GLFWwindow* window, int key, int scancode, int action, int m
 //
 void _glfwInputChar(_GLFWwindow* window, uint32_t codepoint, int mods, GLFWbool plain)
 {
+    assert(window != NULL);
+    assert(mods == (mods & GLFW_MOD_MASK));
+    assert(plain == GLFW_TRUE || plain == GLFW_FALSE);
+
     if (codepoint < 32 || (codepoint > 126 && codepoint < 160))
         return;
 
@@ -317,6 +334,12 @@ void _glfwInputChar(_GLFWwindow* window, uint32_t codepoint, int mods, GLFWbool
 //
 void _glfwInputScroll(_GLFWwindow* window, double xoffset, double yoffset)
 {
+    assert(window != NULL);
+    assert(xoffset > -FLT_MAX);
+    assert(xoffset < FLT_MAX);
+    assert(yoffset > -FLT_MAX);
+    assert(yoffset < FLT_MAX);
+
     if (window->callbacks.scroll)
         window->callbacks.scroll((GLFWwindow*) window, xoffset, yoffset);
 }
@@ -325,6 +348,12 @@ void _glfwInputScroll(_GLFWwindow* window, double xoffset, double yoffset)
 //
 void _glfwInputMouseClick(_GLFWwindow* window, int button, int action, int mods)
 {
+    assert(window != NULL);
+    assert(button >= 0);
+    assert(button <= GLFW_MOUSE_BUTTON_LAST);
+    assert(action == GLFW_PRESS || action == GLFW_RELEASE);
+    assert(mods == (mods & GLFW_MOD_MASK));
+
     if (button < 0 || button > GLFW_MOUSE_BUTTON_LAST)
         return;
 
@@ -345,6 +374,12 @@ void _glfwInputMouseClick(_GLFWwindow* window, int button, int action, int mods)
 //
 void _glfwInputCursorPos(_GLFWwindow* window, double xpos, double ypos)
 {
+    assert(window != NULL);
+    assert(xpos > -FLT_MAX);
+    assert(xpos < FLT_MAX);
+    assert(ypos > -FLT_MAX);
+    assert(ypos < FLT_MAX);
+
     if (window->virtualCursorPosX == xpos && window->virtualCursorPosY == ypos)
         return;
 
@@ -359,6 +394,9 @@ void _glfwInputCursorPos(_GLFWwindow* window, double xpos, double ypos)
 //
 void _glfwInputCursorEnter(_GLFWwindow* window, GLFWbool entered)
 {
+    assert(window != NULL);
+    assert(entered == GLFW_TRUE || entered == GLFW_FALSE);
+
     if (window->callbacks.cursorEnter)
         window->callbacks.cursorEnter((GLFWwindow*) window, entered);
 }
@@ -367,6 +405,10 @@ void _glfwInputCursorEnter(_GLFWwindow* window, GLFWbool entered)
 //
 void _glfwInputDrop(_GLFWwindow* window, int count, const char** paths)
 {
+    assert(window != NULL);
+    assert(count > 0);
+    assert(paths != NULL);
+
     if (window->callbacks.drop)
         window->callbacks.drop((GLFWwindow*) window, count, paths);
 }
@@ -375,7 +417,8 @@ void _glfwInputDrop(_GLFWwindow* window, int count, const char** paths)
 //
 void _glfwInputJoystick(_GLFWjoystick* js, int event)
 {
-    const int jid = (int) (js - _glfw.joysticks);
+    assert(js != NULL);
+    assert(event == GLFW_CONNECTED || event == GLFW_DISCONNECTED);
 
     if (event == GLFW_CONNECTED)
         js->connected = GLFW_TRUE;
@@ -383,13 +426,17 @@ void _glfwInputJoystick(_GLFWjoystick* js, int event)
         js->connected = GLFW_FALSE;
 
     if (_glfw.callbacks.joystick)
-        _glfw.callbacks.joystick(jid, event);
+        _glfw.callbacks.joystick((int) (js - _glfw.joysticks), event);
 }
 
 // Notifies shared code of the new value of a joystick axis
 //
 void _glfwInputJoystickAxis(_GLFWjoystick* js, int axis, float value)
 {
+    assert(js != NULL);
+    assert(axis >= 0);
+    assert(axis < js->axisCount);
+
     js->axes[axis] = value;
 }
 
@@ -397,6 +444,11 @@ void _glfwInputJoystickAxis(_GLFWjoystick* js, int axis, float value)
 //
 void _glfwInputJoystickButton(_GLFWjoystick* js, int button, char value)
 {
+    assert(js != NULL);
+    assert(button >= 0);
+    assert(button < js->buttonCount);
+    assert(value == GLFW_PRESS || value == GLFW_RELEASE);
+
     js->buttons[button] = value;
 }
 
@@ -404,7 +456,18 @@ void _glfwInputJoystickButton(_GLFWjoystick* js, int button, char value)
 //
 void _glfwInputJoystickHat(_GLFWjoystick* js, int hat, char value)
 {
-    const int base = js->buttonCount + hat * 4;
+    int base;
+
+    assert(js != NULL);
+    assert(hat >= 0);
+    assert(hat < js->hatCount);
+
+    // Valid hat values only use the least significant nibble and have at most two bits
+    // set, which can be considered adjacent plus an arbitrary rotation within the nibble
+    assert((value & 0xf0) == 0);
+    assert((value & ((value << 2) | (value >> 2))) == 0);
+
+    base = js->buttonCount + hat * 4;
 
     js->buttons[base + 0] = (value & 0x01) ? GLFW_PRESS : GLFW_RELEASE;
     js->buttons[base + 1] = (value & 0x02) ? GLFW_PRESS : GLFW_RELEASE;
diff --git a/src/monitor.c b/src/monitor.c
index 082140ad..6429493b 100644
--- a/src/monitor.c
+++ b/src/monitor.c
@@ -96,6 +96,10 @@ static GLFWbool refreshVideoModes(_GLFWmonitor* monitor)
 //
 void _glfwInputMonitor(_GLFWmonitor* monitor, int action, int placement)
 {
+    assert(monitor != NULL);
+    assert(action == GLFW_CONNECTED || action == GLFW_DISCONNECTED);
+    assert(placement == _GLFW_INSERT_FIRST || placement == _GLFW_INSERT_LAST);
+
     if (action == GLFW_CONNECTED)
     {
         _glfw.monitorCount++;
@@ -155,6 +159,7 @@ void _glfwInputMonitor(_GLFWmonitor* monitor, int action, int placement)
 //
 void _glfwInputMonitorWindow(_GLFWmonitor* monitor, _GLFWwindow* window)
 {
+    assert(monitor != NULL);
     monitor->window = window;
 }
 
diff --git a/src/window.c b/src/window.c
index 621e2e64..ebbc6dca 100644
--- a/src/window.c
+++ b/src/window.c
@@ -44,6 +44,9 @@
 //
 void _glfwInputWindowFocus(_GLFWwindow* window, GLFWbool focused)
 {
+    assert(window != NULL);
+    assert(focused == GLFW_TRUE || focused == GLFW_FALSE);
+
     if (window->callbacks.focus)
         window->callbacks.focus((GLFWwindow*) window, focused);
 
@@ -73,6 +76,8 @@ void _glfwInputWindowFocus(_GLFWwindow* window, GLFWbool focused)
 //
 void _glfwInputWindowPos(_GLFWwindow* window, int x, int y)
 {
+    assert(window != NULL);
+
     if (window->callbacks.pos)
         window->callbacks.pos((GLFWwindow*) window, x, y);
 }
@@ -82,6 +87,10 @@ void _glfwInputWindowPos(_GLFWwindow* window, int x, int y)
 //
 void _glfwInputWindowSize(_GLFWwindow* window, int width, int height)
 {
+    assert(window != NULL);
+    assert(width >= 0);
+    assert(height >= 0);
+
     if (window->callbacks.size)
         window->callbacks.size((GLFWwindow*) window, width, height);
 }
@@ -90,6 +99,9 @@ void _glfwInputWindowSize(_GLFWwindow* window, int width, int height)
 //
 void _glfwInputWindowIconify(_GLFWwindow* window, GLFWbool iconified)
 {
+    assert(window != NULL);
+    assert(iconified == GLFW_TRUE || iconified == GLFW_FALSE);
+
     if (window->callbacks.iconify)
         window->callbacks.iconify((GLFWwindow*) window, iconified);
 }
@@ -98,6 +110,9 @@ void _glfwInputWindowIconify(_GLFWwindow* window, GLFWbool iconified)
 //
 void _glfwInputWindowMaximize(_GLFWwindow* window, GLFWbool maximized)
 {
+    assert(window != NULL);
+    assert(maximized == GLFW_TRUE || maximized == GLFW_FALSE);
+
     if (window->callbacks.maximize)
         window->callbacks.maximize((GLFWwindow*) window, maximized);
 }
@@ -107,6 +122,10 @@ void _glfwInputWindowMaximize(_GLFWwindow* window, GLFWbool maximized)
 //
 void _glfwInputFramebufferSize(_GLFWwindow* window, int width, int height)
 {
+    assert(window != NULL);
+    assert(width >= 0);
+    assert(height >= 0);
+
     if (window->callbacks.fbsize)
         window->callbacks.fbsize((GLFWwindow*) window, width, height);
 }
@@ -116,6 +135,12 @@ void _glfwInputFramebufferSize(_GLFWwindow* window, int width, int height)
 //
 void _glfwInputWindowContentScale(_GLFWwindow* window, float xscale, float yscale)
 {
+    assert(window != NULL);
+    assert(xscale > 0.f);
+    assert(xscale < FLT_MAX);
+    assert(yscale > 0.f);
+    assert(yscale < FLT_MAX);
+
     if (window->callbacks.scale)
         window->callbacks.scale((GLFWwindow*) window, xscale, yscale);
 }
@@ -124,6 +149,8 @@ void _glfwInputWindowContentScale(_GLFWwindow* window, float xscale, float yscal
 //
 void _glfwInputWindowDamage(_GLFWwindow* window)
 {
+    assert(window != NULL);
+
     if (window->callbacks.refresh)
         window->callbacks.refresh((GLFWwindow*) window);
 }
@@ -132,6 +159,8 @@ void _glfwInputWindowDamage(_GLFWwindow* window)
 //
 void _glfwInputWindowCloseRequest(_GLFWwindow* window)
 {
+    assert(window != NULL);
+
     window->shouldClose = GLFW_TRUE;
 
     if (window->callbacks.close)
@@ -142,6 +171,7 @@ void _glfwInputWindowCloseRequest(_GLFWwindow* window)
 //
 void _glfwInputWindowMonitor(_GLFWwindow* window, _GLFWmonitor* monitor)
 {
+    assert(window != NULL);
     window->monitor = monitor;
 }