diff --git a/mininet/moduledeps.py b/mininet/moduledeps.py
index 7cfe8e8..b21c629 100644
--- a/mininet/moduledeps.py
+++ b/mininet/moduledeps.py
@@ -21,10 +21,12 @@ OF_KMOD = 'ofdatapath'
 OVS_KMOD = 'openvswitch_mod'
 TUN = 'tun'
 
-def moduleDeps( subtract = [], add = [] ):
+def moduleDeps( subtract=None, add=None ):
     """Handle module dependencies.
        subtract: string or list of module names to remove, if already loaded
        add: string or list of module names to add, if not already loaded"""
+    subtract = subtract if subtract is not None else []
+    add = add if add is not None else []
     if type( subtract ) is str:
         subtract = [ subtract ]
     if type( add ) is str:
@@ -51,4 +53,4 @@ def moduleDeps( subtract = [], add = [] ):
                 error( 'Failed to insert ' + mod + '\n' )
                 exit( 1 )
         else:
-            debug( '*** ' + mod + ' already loaded\n' )
\ No newline at end of file
+            debug( '*** ' + mod + ' already loaded\n' )