diff --git a/core/service.go b/core/service.go
index e0e30e3..34c1d53 100644
--- a/core/service.go
+++ b/core/service.go
@@ -660,6 +660,9 @@ func (sh *serviceDescription) authorize(w http.ResponseWriter, r *http.Request)
 			}
 
 			if authtype != newType || uid != newId {
+				if authtype == "firebase" && newType == "guest" {
+					email = fmt.Sprintf("%s@guest.flag", uid)
+				}
 				authtype = newType
 				uid = newId
 			}