diff --git a/javascript/src/utilities/signingUtilities.js b/javascript/src/utilities/signingUtilities.js
index 29628d93ecf2675c39112d34df802f75414b465c..1c94f7c2e78603e7e6cad83cb40dd7a5887b28eb 100644
--- a/javascript/src/utilities/signingUtilities.js
+++ b/javascript/src/utilities/signingUtilities.js
@@ -418,7 +418,7 @@ function createCertificate(certData, issuerData = null) {
         serialNumberView = new Uint8Array(certData.serialNumber);
       } else if (certData.serialNumber instanceof Uint8Array) {
         serialNumberView = certData.serialNumber;
-      } else if (certData.serialNumber instanceof String) {
+      } else if (typeof certData.serialNumber === "string") {
         try {
           serialNumberView = new Uint8Array(hexStringToBytes(certData.serialNumber));
         } catch (ignore) {