diff --git a/src/saml2/client.py b/src/saml2/client.py
index 14fa65f2a..35ca11478 100644
--- a/src/saml2/client.py
+++ b/src/saml2/client.py
@@ -61,6 +61,8 @@ def prepare_for_authenticate(
         :return: session id and AuthnRequest info
         """
 
+        kwargs["nsprefix"] = {'saml': 'urn:oasis:names:tc:SAML:2.0:assertion',
+                              'samlp': 'urn:oasis:names:tc:SAML:2.0:protocol'}
         reqid, negotiated_binding, info = \
             self.prepare_for_negotiated_authenticate(
                 entityid=entityid,