--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -50,7 +50,7 @@ def sortdiff(a, b):
am, bm, = la, lb
# walk over matching lines
- while lb < lenb and la < lenb and a[la] == b[lb] :
+ while lb < lenb and la < lena and a[la] == b[lb] :
la += 1
lb += 1