diff --git a/lib/carto/tree/color.js b/lib/carto/tree/color.js
index 75f393b..dd49d5a 100644
--- a/lib/carto/tree/color.js
+++ b/lib/carto/tree/color.js
@@ -47,7 +47,7 @@ tree.Color.prototype = {
     // channels will spill onto each other. Once we have
     // our result, in the form of an integer triplet,
     // we create a new Color node to hold the result.
-    operate: function(op, other) {
+    operate: function(env, op, other) {
         var result = [];
 
         if (! (other instanceof tree.Color)) {