diff --git a/Lib/urllib.py b/Lib/urllib.py
index a818adf..02edb3b 100644
--- a/Lib/urllib.py
+++ b/Lib/urllib.py
@@ -87,6 +87,7 @@
 	def __init__(self, proxies=None):
 		if proxies is None:
 			proxies = getproxies()
+		assert hasattr(proxies, 'has_key'), "proxies must be a mapping"
 		self.proxies = proxies
 		server_version = "Python-urllib/%s" % __version__
 		self.addheaders = [('User-agent', server_version)]