diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1078,7 +1078,6 @@ class Test(unittest.TestCase):
                                                    b'daemon.pids'))
         env["HGEDITOR"] = ('"' + sys.executable + '"'
                            + ' -c "import sys; sys.exit(0)"')
-        env["HGMERGE"] = "internal:merge"
         env["HGUSER"]   = "test"
         env["HGENCODING"] = "ascii"
         env["HGENCODINGMODE"] = "strict"
@@ -1116,6 +1115,7 @@ class Test(unittest.TestCase):
             'EDITOR',
             'GREP_OPTIONS',
             'HG',
+            'HGMERGE',
             'HGPLAIN',
             'HGPLAINEXCEPT',
             'HGPROF',
@@ -1146,6 +1146,7 @@ class Test(unittest.TestCase):
             hgrc.write(b'[ui]\n')
             hgrc.write(b'slash = True\n')
             hgrc.write(b'interactive = False\n')
+            hgrc.write(b'merge = internal:merge\n')
             hgrc.write(b'mergemarkers = detailed\n')
             hgrc.write(b'promptecho = True\n')
             hgrc.write(b'[defaults]\n')
diff --git a/tests/test-basic.t b/tests/test-basic.t
--- a/tests/test-basic.t
+++ b/tests/test-basic.t
@@ -9,6 +9,7 @@ Create a repository:
   lfs.usercache=$TESTTMP/.cache/lfs
   ui.slash=True
   ui.interactive=False
+  ui.merge=internal:merge
   ui.mergemarkers=detailed
   ui.promptecho=True
   web.address=localhost
diff --git a/tests/test-commandserver.t b/tests/test-commandserver.t
--- a/tests/test-commandserver.t
+++ b/tests/test-commandserver.t
@@ -211,6 +211,7 @@ check that local configs for the cached 
   lfs.usercache=$TESTTMP/.cache/lfs
   ui.slash=True
   ui.interactive=False
+  ui.merge=internal:merge
   ui.mergemarkers=detailed
   ui.foo=bar
   ui.nontty=true
@@ -221,6 +222,7 @@ check that local configs for the cached 
   *** runcommand -R foo showconfig ui defaults
   ui.slash=True
   ui.interactive=False
+  ui.merge=internal:merge
   ui.mergemarkers=detailed
   ui.nontty=true
 #endif