diff --git a/rauth/oauth.py b/rauth/oauth.py
index 5dc44b1..edf8e0c 100644
--- a/rauth/oauth.py
+++ b/rauth/oauth.py
@@ -171,7 +171,8 @@ def __init__(self):
             from Crypto.Signature import PKCS1_v1_5 as p
             self.RSA, self.SHA, self.PKCS1_v1_5 = r, s, p
         except ImportError:  # pragma: no cover
-            raise NotImplementedError('PyCrypto is required for ' + self.NAME)
+            raise NotImplementedError('PyCrypto(dome) is required for '
+                                      + self.NAME)
 
     def sign(self,
              consumer_secret,
@@ -208,7 +209,9 @@ def sign(self,
         # resolve the key
         if is_basestring(consumer_secret):
             consumer_secret = self.RSA.importKey(consumer_secret)
-        if not isinstance(consumer_secret, self.RSA._RSAobj):
+        valid_cls = (getattr(self.RSA, '_RSAobj', False)
+                     or getattr(self.RSA, 'RsaKey', False))
+        if not (valid_cls and isinstance(consumer_secret, valid_cls)):
             raise ValueError('invalid consumer_secret')
 
         # hash the string with RSA-SHA1