diff --git a/Lib/cookielib.py b/Lib/cookielib.py
index 3a716d2..4552412 100644
--- a/Lib/cookielib.py
+++ b/Lib/cookielib.py
@@ -367,7 +367,7 @@
     [[('Basic', None), ('realm', '"foobar"')]]
 
     """
-    assert not isinstance(header_values, basestring)
+    assert not isinstance(header_values, str)
     result = []
     for text in header_values:
         orig_text = text