diff --git a/cert.js b/cert.js
index 44cae8e..c55a760 100644
--- a/cert.js
+++ b/cert.js
@@ -123,7 +123,7 @@ const setup = (jwksOrigin) => {
   return {
     privateKey: forge.pki.privateKeyToPem(privateKey),
     certDer: certDer,
-    thumbPrint: thumbprint,
+    thumbPrint: thumbprint.toString(),
     exponent: bnToB64(exponent),
     modulus: modulus.toString('base64')
   }