diff --git a/test/timestring.js b/test/timestring.js
index 8a54209..39301ee 100644
--- a/test/timestring.js
+++ b/test/timestring.js
@@ -3,7 +3,15 @@ var expect = chai.expect;
 var timestring = require('../timestring');
 
 describe('timestring', function() {
-  it('should expose a method on String.prototype that will attempt to parse the string as a timestring', function(done){
+  it('throws an error when an invalid unit is used in the timestring', function(done) {
+    var ts = new timestring();
+
+    expect(ts.parse.bind(ts, '1g')).to.throw(Error);
+
+    done();
+  });
+
+  it('should expose a method on String.prototype that will parse the string as a timestring', function(done){
     var str = '1min';
 
     // no arguments passed
diff --git a/timestring.js b/timestring.js
index 281809f..bab6110 100644
--- a/timestring.js
+++ b/timestring.js
@@ -12,8 +12,7 @@
 
     // merge default settings with user settings
     settings = settings || {};
-    this.settings = {};
-    for (var d in defaults) { this.settings[d] = defaults[d]; }
+    this.settings = defaults;
     for (var s in settings) { this.settings[s] = settings[s]; }
 
     // time units
@@ -56,8 +55,8 @@
         }
       }
 
-      // throw exception if invalid unit is passed
-      throw 'The unit [' + unit + '] is not supported by timestring';
+      // throw error if invalid unit was passed
+      throw new Error('The unit [' + unit + '] is not supported by timestring');
     }
 
     // convert a value to a specific unit