diff --git a/soundchanger/change.py b/soundchanger/change.py
index e19126a..c4d1fcd 100644
--- a/soundchanger/change.py
+++ b/soundchanger/change.py
@@ -1,7 +1,7 @@
 import re
 
 
-def apply(change, string, categories=None, apply=True):
+def apply(change, string, categories={}, apply=True):
     """Apply a sound change to a given string"""
 
     if not apply: