--- a/mercurial/merge.py	Sun Oct 08 21:47:14 2017 +0200
+++ b/mercurial/merge.py	Sun Oct 08 21:48:40 2017 +0200
@@ -603,7 +603,7 @@
         self._results[f] = 0, 'g'
 
 def _getcheckunknownconfig(repo, section, name):
-    config = repo.ui.config(section, name, default='abort')
+    config = repo.ui.config(section, name)
     valid = ['abort', 'ignore', 'warn']
     if config not in valid:
         validstr = ', '.join(["'" + v + "'" for v in valid])