_box-sizing.scss in susy-2.2.4

- old
+ new

@@ -6,12 +6,14 @@
 // Check for an existing support mixin, or provide a simple fallback.
 // - $model: <box-sizing>
 @mixin susy-box-sizing(
   $model: content-box
 ) {
-  @if susy-support(box-sizing, (mixin: box-sizing), $warn: false) {
-    @include box-sizing($model);
-  } @else {
-    $prefix: (moz, webkit, official);
-    @include susy-prefix(box-sizing, $model, $prefix);
+  @if $model {
+    @if susy-support(box-sizing, (mixin: box-sizing), $warn: false) {
+      @include box-sizing($model);
+    } @else {
+      $prefix: (moz, webkit, official);
+      @include susy-prefix(box-sizing, $model, $prefix);
+    }
   }
 }