diff --git a/Lib/warnings.py b/Lib/warnings.py
index d9e6e44..132a34d 100644
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -278,6 +278,9 @@
                 _show_warning(message, category, filename, lineno)
             else:
                 warn(showwarning_msg, DeprecationWarning)
+    if not callable(showwarning):
+        raise TypeError("warnings.showwarning() must be set to a "
+                        "function or method")
     # Print message and context
     showwarning(message, category, filename, lineno)