diff --git a/lib/collection.js b/lib/collection.js
index 74fbd58..99b4c26 100644
--- a/lib/collection.js
+++ b/lib/collection.js
@@ -359,11 +359,7 @@ _.extend(Collection.prototype, Events, {
 
 });
 
-var underscoreMethodExists = function(method) {
-  return typeof _[method] === 'function';
-};
-
-if (underscoreMethodExists('each')) {
+if (utilExists('each')) {
   // Underscore methods that we want to implement on the Collection.
   // 90% of the core usefulness of Backbone Collections is actually implemented
   // right here:
@@ -375,30 +371,26 @@ if (underscoreMethodExists('each')) {
     'lastIndexOf', 'isEmpty', 'chain'];
 
   // Mix in each Underscore method as a proxy to `Collection#models`.
-  methods
-    .filter(underscoreMethodExists)
-    .forEach(function(method) {
-      Collection.prototype[method] = function() {
-        var args = slice.call(arguments);
-        args.unshift(this.models);
-        return _[method].apply(_, args);
-      };
-    });
+  methods.filter(utilExists).forEach(function(method) {
+    Collection.prototype[method] = function() {
+      var args = slice.call(arguments);
+      args.unshift(this.models);
+      return _[method].apply(_, args);
+    };
+  });
 
   // Underscore methods that take a property name as an argument.
   var attributeMethods = ['groupBy', 'countBy', 'sortBy'];
 
   // Use attributes instead of properties.
-  attributeMethods
-    .filter(underscoreMethodExists)
-    .forEach(function(method) {
-      Collection.prototype[method] = function(value, context) {
-        var iterator = typeof value === 'function' ? value : function(model) {
-          return model.get(value);
-        };
-        return _[method](this.models, iterator, context);
+  attributeMethods.filter(utilExists).forEach(function(method) {
+    Collection.prototype[method] = function(value, context) {
+      var iterator = typeof value === 'function' ? value : function(model) {
+        return model.get(value);
       };
-    });
+      return _[method](this.models, iterator, context);
+    };
+  });
 } else {
   ['forEach', 'map', 'filter', 'some', 'every', 'reduce', 'reduceRight',
     'indexOf', 'lastIndexOf'].forEach(function(method) {
diff --git a/lib/header.js b/lib/header.js
index 2131129..b89adf8 100644
--- a/lib/header.js
+++ b/lib/header.js
@@ -133,3 +133,8 @@
       model.trigger('error', model, resp, options);
     };
   };
+
+  // Checker for utility methods. Useful for custom builds.
+  var utilExists = function(method) {
+    return typeof _[method] === 'function';
+  };
diff --git a/lib/model.js b/lib/model.js
index 3fb52bd..46025e0 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -337,7 +337,7 @@ if (_.keys) {
   var modelMethods = ['keys', 'values', 'pairs', 'invert', 'pick', 'omit'];
 
   // Mix in each Underscore method as a proxy to `Model#attributes`.
-  modelMethods.forEach(function(method) {
+  modelMethods.filter(utilExists).forEach(function(method) {
     Model.prototype[method] = function() {
       var args = slice.call(arguments);
       args.unshift(this.attributes);