diff --git a/frameworks/compass/stylesheets/compass/css3/_background-size.scss b/frameworks/compass/stylesheets/compass/css3/_background-size.scss
index 84c11097..a9b7a04f 100644
--- a/frameworks/compass/stylesheets/compass/css3/_background-size.scss
+++ b/frameworks/compass/stylesheets/compass/css3/_background-size.scss
@@ -9,6 +9,8 @@ $default-background-size: 100% auto !default;
 // * percentages are relative to the background-origin (default = padding-box)
 // * mixin defaults to: `$default-background-size`
 @mixin background-size($size: $default-background-size) {
-  $size: unquote($size);
+  @if type-of($size) == string {
+    $size: unquote($size);
+  }
   @include experimental(background-size, $size, -moz, -webkit, -o, not -ms, not -khtml);
 }