diff --git a/.changeset/fluffy-pens-prove.md b/.changeset/fluffy-pens-prove.md
new file mode 100644
index 0000000000..0bd7d9baf3
--- /dev/null
+++ b/.changeset/fluffy-pens-prove.md
@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Updated `frameHandler` to return `undefined` when using the redirect flow instead of returning `postMessageReponse` which was causing errors
diff --git a/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts b/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts
index 5f51b2d301..fb0750f0cb 100644
--- a/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts
+++ b/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts
@@ -187,6 +187,7 @@ export class OAuthAdapter implements AuthProviderRouteHandlers {
           );
         }
         res.redirect(state.redirectUrl);
+        return undefined;
       }
       // post message back to popup if successful
       return postMessageResponse(res, appOrigin, responseObj);