diff --git a/Lib/inspect.py b/Lib/inspect.py
index bb46ea6..4e17f1b 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -483,7 +483,7 @@
         return sys.modules.get(modulesbyfile[file])
     # Update the filename to module name cache and check yet again
     # Copy sys.modules in order to cope with changes while iterating
-    for modname, module in sys.modules.items():
+    for modname, module in list(sys.modules.items()):
         if ismodule(module) and hasattr(module, '__file__'):
             f = module.__file__
             if f == _filesbymodname.get(modname, None):