diff -r 40852b4b910c -r 4cdaf1adafc8 mercurial/merge.py
--- a/mercurial/merge.py	Fri Sep 24 12:00:55 2010 +0200
+++ b/mercurial/merge.py	Fri Sep 24 12:46:54 2010 -0500
@@ -496,7 +496,7 @@
                 raise util.Abort(_("outstanding uncommitted changes "
                                    "(use 'hg status' to list changes)"))
         elif not overwrite:
-            if pa in (p1, p2): # linear
+            if pa == p1 or pa == p2: # linear
                 pass # all good
             elif wc.files() or wc.deleted():
                 raise util.Abort(_("crosses branches (use 'hg merge' to merge "