diff --git a/lib/dkim/dkim-signer.js b/lib/dkim/dkim-signer.js
index 7354eb8..6996432 100644
--- a/lib/dkim/dkim-signer.js
+++ b/lib/dkim/dkim-signer.js
@@ -266,7 +266,7 @@ class DkimSigner extends MessageParser {
                     {},
                     signatureData,
                     {
-                        instance: this.arc?.instance || 1, // ARC only
+                        instance: 'instance' in this.arc && (this.arc.instance || 1), // ARC only
                         algorithm,
                         canonicalization: this.getCanonicalization(signatureData).canonicalization,