Index: pyyaml/trunk/lib/yaml/resolver.py
===================================================================
--- pyyaml/trunk/lib/yaml/resolver.py	(revision 137)
+++ pyyaml/trunk/lib/yaml/resolver.py	(revision 155)
@@ -26,4 +26,6 @@
         if not 'yaml_implicit_resolvers' in cls.__dict__:
             cls.yaml_implicit_resolvers = cls.yaml_implicit_resolvers.copy()
+        if first is None:
+            first = [None]
         for ch in first:
             cls.yaml_implicit_resolvers.setdefault(ch, []).append((tag, regexp))