1 files changed, 2 insertions, 2 deletions
diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index 8d08e8b413..d3d1af8dec 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -574,8 +574,8 @@ class PurePath(object):
             if isinstance(a, PurePath):
                 parts += a._parts
             elif isinstance(a, str):
-                # Assuming a str
-                parts.append(a)
+                # Force-cast str subclasses to str (issue #21127)
+                parts.append(str(a))
             else:
                 raise TypeError(
                     "argument should be a path or str object, not %r"