diff --git a/mercurial/similar.py b/mercurial/similar.py
--- a/mercurial/similar.py
+++ b/mercurial/similar.py
@@ -63,7 +63,8 @@
             equal += len(line)
 
     lengths = len(text) + len(orig)
-    return equal * 2.0 / lengths
+    if lengths > 0:
+        return equal * 2.0 / lengths
 
 def score(fctx1, fctx2):
     return _score(fctx1, _ctxdata(fctx2))