diff --git a/files/acme_renew_cert.py b/files/acme_renew_cert.py
index 261127f..29ce4d6 100755
--- a/files/acme_renew_cert.py
+++ b/files/acme_renew_cert.py
@@ -283,7 +283,7 @@ class Domain(PrivateKey):
             for subject_domain in subject_domains:
                 _, domain = str(subject_domain).split(':')
                 domain_names.add(domain)
-        domain_names_diff = domain_names & set([self.name] + self.alt_names)
+        domain_names_diff = domain_names ^ set([self.name] + self.alt_names)
         if len(domain_names_diff) > 0:
             logging.warning(f"Differences found in CSR: {domain_names_diff}")
             csr = self.create_csr()