diff --git a/lib/actions/grants/refresh_token.js b/lib/actions/grants/refresh_token.js
index a8a24947a..9b61687b0 100644
--- a/lib/actions/grants/refresh_token.js
+++ b/lib/actions/grants/refresh_token.js
@@ -85,7 +85,7 @@ module.exports.handler = function getRefreshTokenHandler(provider) {
           gty: refreshToken.gty,
         });
 
-        if (!refreshToken.gty.endsWith(gty)) {
+        if (refreshToken.gty && !refreshToken.gty.endsWith(gty)) {
           refreshToken.gty = `${refreshToken.gty} ${gty}`;
         }
 
@@ -116,7 +116,7 @@ module.exports.handler = function getRefreshTokenHandler(provider) {
       at.setS256Thumbprint(cert);
     }
 
-    if (!at.gty.endsWith(gty)) {
+    if (at.gty && !at.gty.endsWith(gty)) {
       at.gty = `${at.gty} ${gty}`;
     }