diff --git a/.changeset/all-my-auth-wanna-ride-today.md b/.changeset/all-my-auth-wanna-ride-today.md
new file mode 100644
index 0000000000..40537c30f4
--- /dev/null
+++ b/.changeset/all-my-auth-wanna-ride-today.md
@@ -0,0 +1,5 @@
+---
+'@backstage/core-components': patch
+---
+
+Fixed a bug that could prevent auth from working when using the `guest` or `custom` auth providers.
diff --git a/packages/core-components/src/layout/SignInPage/providers.tsx b/packages/core-components/src/layout/SignInPage/providers.tsx
index 5e7ab7b7be..e456a6948b 100644
--- a/packages/core-components/src/layout/SignInPage/providers.tsx
+++ b/packages/core-components/src/layout/SignInPage/providers.tsx
@@ -177,7 +177,10 @@ export const useSignInProviders = (
         };
 
         const handleSignInStarted = () => {
-          localStorage.setItem(PROVIDER_STORAGE_KEY, provider.config!.id);
+          localStorage.setItem(
+            PROVIDER_STORAGE_KEY,
+            provider?.config?.id || provider.id,
+          );
         };
 
         const handleSignInFailure = () => {