don't allow UnrecognizedExtension with get_extension_for_class

1 files changed, 7 insertions, 0 deletions

diff --git a/src/cryptography/x509/extensions.py b/src/cryptography/x509/extensions.py
index 0c5b5523..f7b5d7f5 100644
--- a/src/cryptography/x509/extensions.py
+++ b/src/cryptography/x509/extensions.py
@@ -91,6 +91,13 @@ class Extensions(object):
         raise ExtensionNotFound("No {0} extension was found".format(oid), oid)
 
     def get_extension_for_class(self, extclass):
+        if extclass is UnrecognizedExtension:
+            raise TypeError(
+                "UnrecognizedExtension can't be used with "
+                "get_extension_for_class because more than one instance of the"
+                " class may be present."
+            )
+
         for ext in self:
             if isinstance(ext.value, extclass):
                 return ext