Index: django/db/models/fields/__init__.py
===================================================================
--- django/db/models/fields/__init__.py	(revision 5416)
+++ django/db/models/fields/__init__.py	(working copy)
@@ -610,7 +610,10 @@
         if value < 0:
             num_chars += 1
 
-        return "%.*f" % (self.decimal_places, value)
+        if value is None:
+            return ''
+        else:
+            return "%.*f" % (self.decimal_places, value)
 
     def get_db_prep_save(self, value):
         if value is not None: