diff --git a/pykolab/auth/ldap/__init__.py b/pykolab/auth/ldap/__init__.py
--- a/pykolab/auth/ldap/__init__.py
+++ b/pykolab/auth/ldap/__init__.py
@@ -2112,8 +2112,9 @@
 
         # If we have a 1:1 match, continue as planned
         for naming_context in naming_contexts:
-            if self.domain_rootdns[domain].endswith(naming_context):
-                return naming_context
+            if domain in self.domain_rootdns:
+                if self.domain_rootdns[domain].endswith(naming_context):
+                    return naming_context
 
     def _primary_domain_for_naming_context(self, naming_context):
         self._bind()