Index: src/wp-includes/js/customize-base.js
===================================================================
--- src/wp-includes/js/customize-base.js	(revision 26694)
+++ src/wp-includes/js/customize-base.js	(working copy)
@@ -255,15 +255,15 @@
 
 		/**
 		 * Internal recursive comparison function for `isEqual`.
-		 * Copied from Underscore.js.
+		 * Modified version from Underscore.js v1.5.2.
 		 */
 		/* jshint ignore:start */
 		_eq: function(a, b, aStack, bStack) {
 			if (a === b) return a !== 0 || 1 / a == 1 / b;
 			if (a == null || b == null) return a === b;
 
-			var className = toString.call(a);
-			if (className != toString.call(b)) return false;
+			var className = Object.prototype.toString.call(a);
+			if (className != Object.prototype.toString.call(b)) return false;
 			switch (className) {
 				case '[object String]':
 					return a == String(b);