diff -r 5c736ba5dc27 -r 68aedad4c11c mercurial/pure/bdiff.py
--- a/mercurial/pure/bdiff.py	Mon Nov 16 16:38:57 2020 +0100
+++ b/mercurial/pure/bdiff.py	Sat Nov 07 16:45:58 2020 -0800
@@ -51,7 +51,10 @@
                 shift += 1
         r.append((a1, b1, l1 + shift))
         prev = a2 + shift, b2 + shift, l2 - shift
-    r.append(prev)
+
+    if prev is not None:
+        r.append(prev)
+
     return r