diff --git a/lib/avltree.js b/lib/avltree.js
index 10b2b82..c821c2f 100644
--- a/lib/avltree.js
+++ b/lib/avltree.js
@@ -3,7 +3,6 @@
  */
 var BinarySearchTree = require('./bst')
   , customUtils = require('./customUtils')
-  , util = require('util')
   , _ = require('underscore')
   ;
 
@@ -46,7 +45,7 @@ function _AVLTree (options) {
 /**
  * Inherit basic functions from the basic binary search tree
  */
-util.inherits(_AVLTree, BinarySearchTree);
+customUtils.inherits(_AVLTree, BinarySearchTree);
 
 /**
  * Keep a pointer to the internal tree constructor for testing purposes
diff --git a/lib/customUtils.js b/lib/customUtils.js
index 611b52f..bc30496 100644
--- a/lib/customUtils.js
+++ b/lib/customUtils.js
@@ -39,3 +39,32 @@ function defaultCheckValueEquality (a, b) {
   return a === b;
 }
 module.exports.defaultCheckValueEquality = defaultCheckValueEquality;
+
+/**
+ * Inherit the prototype methods from one constructor into another.
+ *
+ * The Function.prototype.inherits from lang.js rewritten as a standalone
+ * function (not on Function.prototype). NOTE: If this file is to be loaded
+ * during bootstrapping this function needs to be rewritten using some native
+ * functions as prototype setup using normal JavaScript does not work as
+ * expected during bootstrapping (see mirror.js in r114903).
+ *
+ * @param {function} ctor Constructor function which needs to inherit the
+ *     prototype.
+ * @param {function} superCtor Constructor function to inherit prototype from.
+ * @throws {TypeError} Will error if either constructor is null, or if
+ *     the super constructor lacks a prototype.
+ */
+function inherits(ctor, superCtor) {
+  if (ctor === undefined || ctor === null)
+    throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'ctor', 'function');
+  if (superCtor === undefined || superCtor === null)
+    throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'superCtor', 'function');
+  if (superCtor.prototype === undefined) {
+    throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'superCtor.prototype',
+                               'function');
+  }
+  ctor.super_ = superCtor;
+  Object.setPrototypeOf(ctor.prototype, superCtor.prototype);
+}
+module.exports.inherits = inherits;