diff --git a/mercurial/rcutil.py b/mercurial/rcutil.py
--- a/mercurial/rcutil.py
+++ b/mercurial/rcutil.py
@@ -91,7 +91,8 @@
                 continue
             with resourceutil.open_resource(b'defaultrc', name) as fp:
                 cfg = config.config()
-                cfg.read(pycompat.sysbytes(fp.name), fp=fp)
+                source = util.getattr(fp, 'name', b'<internal>/' + name)
+                cfg.read(pycompat.sysbytes(source), fp=fp)
                 for section in cfg:
                     for name, value in cfg.items(section):
                         source = cfg.source(section, name)