diff --git a/phpseclib/System/SSH/Agent.php b/phpseclib/System/SSH/Agent.php
index 8ad83ab9..061aad6f 100644
--- a/phpseclib/System/SSH/Agent.php
+++ b/phpseclib/System/SSH/Agent.php
@@ -61,6 +61,11 @@ class System_SSH_Agent
     const SSH_AGENT_SIGN_RESPONSE = 14;
     /**#@-*/
 
+    /**
+     * Unused
+     */
+    const SSH_AGENT_FAILURE = 5;
+
     /**
      * Socket Resource
      *