diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -344,6 +344,12 @@ coreconfigitem('http_proxy', 'passwd',
 coreconfigitem('http_proxy', 'user',
     default=None,
 )
+coreconfigitem('merge', 'checkunknown',
+    default='abort',
+)
+coreconfigitem('merge', 'checkignored',
+    default='abort',
+)
 coreconfigitem('merge', 'followcopies',
     default=True,
 )
diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -603,7 +603,7 @@ class mergestate(object):
         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])