diff --git a/lib/matplotlib/testing/decorators.py b/lib/matplotlib/testing/decorators.py
index 7930e0b47935..6af18dd59609 100644
--- a/lib/matplotlib/testing/decorators.py
+++ b/lib/matplotlib/testing/decorators.py
@@ -461,7 +461,7 @@ def find_dotted_module(module_name, path=None):
                         file.close()
                 except ImportError:
                     # assume namespace package
-                    path = sys.modules[sub_mod].__path__
+                    path = list(sys.modules[sub_mod].__path__)
                     res = None, path, None
             return res