--- a/mercurial/merge.py	Mon Nov 30 18:47:33 2015 +0000
+++ b/mercurial/merge.py	Mon Nov 30 10:05:09 2015 -0800
@@ -288,6 +288,8 @@
 
     @util.propertycache
     def otherctx(self):
+        if self._other is None:
+            raise RuntimeError("localctx accessed but self._local isn't set")
         return self._repo[self._other]
 
     def active(self):