Index: mercurial/merge.py
===================================================================
--- mercurial/merge.py
+++ mercurial/merge.py
@@ -646,6 +646,14 @@
     return config
 
 def _checkunknownfile(repo, wctx, mctx, f, f2=None):
+    if wctx.isinmemory():
+        # Nothing to do in IMM because nothing in the "working copy" can be an
+        # unknown file.
+        #
+        # Note that we should bail out here, not in ``_checkunknownfiles()``,
+        # because that function does other useful work.
+        return False
+
     if f2 is None:
         f2 = f
     return (repo.wvfs.audit.check(f)