diff --git a/src/fetch-wrapper.ts b/src/fetch-wrapper.ts
index 1517ad5..3a11d56 100644
--- a/src/fetch-wrapper.ts
+++ b/src/fetch-wrapper.ts
@@ -158,7 +158,7 @@ export default class OAuth2 {
         grant_type: 'refresh_token',
         refresh_token: previousToken.refreshToken
       };
-      if ((this.options as any).clientSecret !== undefined) {
+      if ((this.options as any).clientSecret === undefined) {
         // If there is no secret, it means we need to send the clientId along
         // in the body.
         body.client_id = this.options.clientId;