// claim is only considered stable when subject and iss are used together
	// this is to ensure that ParentUser doesn't change and we get to use
	// parentUser as per the requirements for service accounts for OpenID
	// based logins.
	var subFromToken string
	if v, ok := claims[subClaim]; ok {
		subFromToken, _ = v.(string)
	}

	if subFromToken == "" {
		writeSTSErrorResponse(ctx, w, ErrSTSInvalidParameterValue,