diff --git a/tests/test-config-env.py b/tests/test-config-env.py
--- a/tests/test-config-env.py
+++ b/tests/test-config-env.py
@@ -6,6 +6,7 @@
 
 from mercurial import (
     encoding,
+    extensions,
     rcutil,
     ui as uimod,
     util,
@@ -35,9 +36,10 @@
     return [join(b'userrc')]
 
 
+extensions.wrapfunction(rcutil, 'defaultrcpath', lambda orig: [])
+
 rcutil.systemrcpath = systemrcpath
 rcutil.userrcpath = userrcpath
-os.path.isdir = lambda x: False  # hack: do not load default.d/*.rc
 
 # utility to print configs
 def printconfigs(env):