diff --git a/doc/autobuild.py b/doc/autobuild.py
index 362816dcd..932877fc0 100644
--- a/doc/autobuild.py
+++ b/doc/autobuild.py
@@ -160,7 +160,7 @@ template_examples_ref = \
 
 def extract_summary_line(doc):
     if doc is None:
-        return
+        return ""
     for line in doc.split('\n'):
         line = line.strip()
         # don't take empty line