diff --git a/Lib/pickle.py b/Lib/pickle.py
index bfa3c0361b73c6..d7192fea8e396d 100644
--- a/Lib/pickle.py
+++ b/Lib/pickle.py
@@ -307,7 +307,9 @@ def whichmodule(obj, name):
     # Protect the iteration by using a list copy of sys.modules against dynamic
     # modules that trigger imports of other modules upon calls to getattr.
     for module_name, module in list(sys.modules.items()):
-        if module_name == '__main__' or module is None:
+        if (module_name == '__main__'
+             or module_name == '__mp_main__'  # bpo-42406
+             or module is None):
             continue
         try:
             if _getattribute(module, name)[0] is obj: