Invalid output from box-shadow mixin

[oncode]

The blank/default $inset parameter in the box-shadow mixin (#1) is producing following invalid output:

-webkit-box-shadow: "" 2px 2px 5px rgba(0, 0, 0, 0.4);
   -moz-box-shadow: "" 2px 2px 5px rgba(0, 0, 0, 0.4);
           box-shadow: "" 2px 2px 5px rgba(0, 0, 0, 0.4);

(sass version: 3.2.9)

I suggest to check if the string is empty:

/* BOX SHADOW */
@mixin box-shadow($x: 2px, $y: 2px, $blur: 5px, $color: rgba(0,0,0,.4), $inset: "") {
  @if ($inset != "") {
    -webkit-box-shadow: $inset $x $y $blur $color;
    -moz-box-shadow:    $inset $x $y $blur $color;
    box-shadow:         $inset $x $y $blur $color;
  } @else {
    -webkit-box-shadow: $x $y $blur $color;
    -moz-box-shadow:    $x $y $blur $color;
    box-shadow:         $x $y $blur $color;
  }
}