diff --git a/underscore.js b/underscore.js
index 6c06acbab..6e01bbad1 100644
--- a/underscore.js
+++ b/underscore.js
@@ -25,6 +25,7 @@
 
   // Create quick reference variables for speed access to core prototypes.
   var slice            = ArrayProto.slice,
+      concat           = ArrayProto.concat,
       unshift          = ArrayProto.unshift,
       toString         = ObjProto.toString,
       hasOwnProperty   = ObjProto.hasOwnProperty;