diff --git a/lib/less/node/alpha.js b/lib/less/node/alpha.js
index 91d8729..a2da3d0 100644
--- a/lib/less/node/alpha.js
+++ b/lib/less/node/alpha.js
@@ -5,6 +5,6 @@ tree.Alpha = function Alpha(val) {
 };
 tree.Alpha.prototype = {
     toCSS: function () {
-        return "alpha(opacity=" + (this.value.toCSS ? this.value.toCSS() : this.value + ")");
+        return "alpha(opacity=" + (this.value.toCSS ? this.value.toCSS() : this.value) + ")";
     }
 };