diff --git a/phpseclib/Crypt/Hash.php b/phpseclib/Crypt/Hash.php
index 9db55264..ae6f2c8c 100644
--- a/phpseclib/Crypt/Hash.php
+++ b/phpseclib/Crypt/Hash.php
@@ -112,6 +112,15 @@ class Hash
      */
     var $key = false;
 
+    /**
+     * Computed Key
+     *
+     * @see self::_computeKey()
+     * @var string
+     * @access private
+     */
+    var $computedKey = false;
+
     /**
      * Outer XOR (Internal HMAC)
      *
@@ -183,6 +192,11 @@ class Hash
      */
     function _computeKey()
     {
+        if ($this->key === false) {
+            $this->computedKey = false;
+            return;
+        }
+
         if (strlen($this->key) <= $this->b) {
             $this->computedKey = $this->key;
             return;