diff --git a/pyas2lib/as2.py b/pyas2lib/as2.py
index 67ee322..ff84f56 100644
--- a/pyas2lib/as2.py
+++ b/pyas2lib/as2.py
@@ -978,8 +978,9 @@ def detect_mdn(self):
             if part.get_content_type() == "message/disposition-notification":
                 mdn = part.get_payload()[0]
                 message_id = mdn.get("Original-Message-ID").strip("<>")
-                if mdn.get("Original-Recipient"):
-                    message_recipient = mdn.get("Original-Recipient").split(";")[1].strip()
-                else:
-                    message_recipient = mdn.get("Final-Recipient").split(";")[1].strip()
+                message_recipient = None
+                if "Original-Recipient" in mdn:
+                    message_recipient = mdn["Original-Recipient"].split(";")[1].strip()
+                elif "Final-Recipient" in mdn:
+                    message_recipient = mdn["Final-Recipient"].split(";")[1].strip()
         return message_id, message_recipient
diff --git a/pyas2lib/tests/test_advanced.py b/pyas2lib/tests/test_advanced.py
index 4e9650d..5947194 100644
--- a/pyas2lib/tests/test_advanced.py
+++ b/pyas2lib/tests/test_advanced.py
@@ -473,7 +473,7 @@ def setUp(self):
             verify_cert_ca=self.sb2bi_public_ca,
             encrypt_cert=self.sb2bi_public_key,
             encrypt_cert_ca=self.sb2bi_public_ca,
-            validate_certs=False
+            validate_certs=False,
         )
         self.partner.load_verify_cert()
         self.partner.load_encrypt_cert()