diff --git a/Doc/tools/refcounts.py b/Doc/tools/refcounts.py
index 90f47d0..d7c761b 100644
--- a/Doc/tools/refcounts.py
+++ b/Doc/tools/refcounts.py
@@ -32,6 +32,8 @@
             # blank lines and comments
             continue
         parts = string.split(line, ":", 4)
+        if len(parts) != 5:
+            raise ValueError("Not enough fields in " + `line`)
         function, type, arg, refcount, comment = parts
         if refcount == "null":
             refcount = None