Fix bug to allow for null value in $speed list values

README.md

@@ -2,6 +2,9 @@
 
 The SCSS Typewriter is a fully functional typewriter mixin for SCSS.
 
+![demonstration of SCSS typewriter in action](http://i.imgur.com/OUkLi.gif)
+<div align="center">[Live Demo](http://i.imgur.com/OUkLi.gif)</div>
+
 ## Syntax
 
 ```scss
@@ -66,7 +69,7 @@ Properties of the `$options` map can only be overwridden using another object of
 ```
 **Type two strings, adjust the speed of `type` and `pause-deleted` properties** (2 methods)
 ```scss
-@include typewriter("String 1", "String 2", [.1, null, null, .5]); 
+@include typewriter("String 1", "String 2", [.1, null, null, .5]);
 ```
 ```scss
 @include typewriter("String 1", "String 2", (type: .1, pause-deleted: .5));

typewriter.scss

@@ -73,7 +73,7 @@ $__typewriter-id: 0;
 					@error "The speed list argument only accepts 4 numbers when used a list.";
 				}
 				@for $i from 1 through length($parameter) {
-					@if type-of(nth($parameter, $i)) != number {
+					@if nth($parameter, $i) != null and type-of(nth($parameter, $i)) != number {
 						@error "The value #{$value} is not a number.";
 					}
 					@if nth($parameter, $i) != null {
@@ -128,7 +128,7 @@ $__typewriter-id: 0;
 		$animationName: #{map-get($options, name)};
 	} @else {
 		$animationName: typewriter-#{$__typewriter-id};
-		$__typewriter-id: $__typewriter-id + 1;
+		$__typewriter-id: $__typewriter-id + 1 !global;
 	}
 	$charCount: 0;
 	$durTotal: 0;