Index: pyyaml/trunk/lib/yaml/serializer.py
===================================================================
--- pyyaml/trunk/lib/yaml/serializer.py	(revision 137)
+++ pyyaml/trunk/lib/yaml/serializer.py	(revision 139)
@@ -68,7 +68,12 @@
                     self.anchor_node(item)
             elif isinstance(node, MappingNode):
-                for key in node.value:
-                    self.anchor_node(key)
-                    self.anchor_node(node.value[key])
+                if hasattr(node.value, 'keys'):
+                    for key in node.value.keys():
+                        self.anchor_node(key)
+                        self.anchor_node(node.value[key])
+                else:
+                    for key, value in node.value:
+                        self.anchor_node(key)
+                        self.anchor_node(value)
 
     def generate_anchor(self, node):