diff --git a/lib/less/tree/mixin.js b/lib/less/tree/mixin.js
index 9ac651a6..85026db2 100644
--- a/lib/less/tree/mixin.js
+++ b/lib/less/tree/mixin.js
@@ -48,7 +48,7 @@ tree.mixin.Definition.prototype = {
         for (var i = 0, val; i < this.params.length; i++) {
             if (this.params[i].name) {
                 if (val = (args && args[i]) || this.params[i].value) {
-                    frame.rules.unshift(new(tree.Rule)(this.params[i].name, val));
+                    frame.rules.unshift(new(tree.Rule)(this.params[i].name, val.eval(env)));
                 } else {
                     throw new(Error)("wrong number of arguments for " + this.name);
                 }