diff --git a/hgext/histedit.py b/hgext/histedit.py
--- a/hgext/histedit.py
+++ b/hgext/histedit.py
@@ -1124,7 +1124,7 @@
     if next:
         index += 1
     else:
-        index += 1
+        index -= 1
     changeaction(state, pos, KEY_LIST[index % len(KEY_LIST)])
 
 def changeview(state, delta, unit):
diff --git a/mercurial/similar.py b/mercurial/similar.py
--- a/mercurial/similar.py
+++ b/mercurial/similar.py
@@ -63,6 +63,7 @@
             equal += len(line)
 
     lengths = len(text) + len(orig)
+    if lengths > 0:
     return equal * 2.0 / lengths
 
 def score(fctx1, fctx2):