diff --git a/IPython/zmq/parallel/client.py b/IPython/zmq/parallel/client.py
index bde8d5b..99363f2 100644
--- a/IPython/zmq/parallel/client.py
+++ b/IPython/zmq/parallel/client.py
@@ -231,7 +231,7 @@ class Client(object):
                 password = getpass("SSH Password for %s: "%sshserver)
         ssh_kwargs = dict(keyfile=sshkey, password=password, paramiko=paramiko)
         
-        if os.path.isfile(exec_key):
+        if exec_key is not None and os.path.isfile(exec_key):
             arg = 'keyfile'
         else:
             arg = 'key'