diff --git a/modoboa/admin/lib.py b/modoboa/admin/lib.py
index 0b1cc30c0..d378322c1 100644
--- a/modoboa/admin/lib.py
+++ b/modoboa/admin/lib.py
@@ -178,7 +178,7 @@ def get_authoritative_server(domain, resolver=None):
 
     while True:
         try:
-            answers = dns.resolver.resolve(dnsname, "NS")
+            answers = resolver.resolve(dnsname, "NS")
             if answers:
                 first_answer = answers[0]
                 if hasattr(first_answer, "target"):