diff --git a/Lib/argparse.py b/Lib/argparse.py
index cc3e374..67bbef2 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1709,9 +1709,12 @@
         return args
 
     def parse_known_args(self, args=None, namespace=None):
-        # args default to the system args
         if args is None:
+            # args default to the system args
             args = _sys.argv[1:]
+        else:
+            # make sure that args are mutable
+            args = list(args)
 
         # default Namespace built from parser defaults
         if namespace is None: