diff --git a/imagekit/registry.py b/imagekit/registry.py
index 4147f87..ccc9b7e 100644
--- a/imagekit/registry.py
+++ b/imagekit/registry.py
@@ -131,7 +131,7 @@ class Register(object):
     Register specs and sources.
 
     """
-    def spec(self, id, spec):
+    def spec(self, spec, id=None):
         generator_registry.register(spec, id)
 
     def sources(self, spec_id, sources):