diff -r 91d1fc2a3a14 -r 33b6843fdf8a src/main/java/de/uapcore/lightpit/dao/postgres/PGUserDao.java
--- a/src/main/java/de/uapcore/lightpit/dao/postgres/PGUserDao.java	Fri May 22 17:26:27 2020 +0200
+++ b/src/main/java/de/uapcore/lightpit/dao/postgres/PGUserDao.java	Fri May 22 21:23:57 2020 +0200
@@ -58,7 +58,7 @@
         update = connection.prepareStatement("update lpit_user set lastname = ?, givenname = ?, mail = ? where userid = ?");
     }
 
-    public User mapColumns(ResultSet result) throws SQLException {
+    private User mapColumns(ResultSet result) throws SQLException {
         final int id = result.getInt("userid");
         if (id == 0) return null;
         final var user = new User(id);
@@ -81,6 +81,7 @@
 
     @Override
     public boolean update(User instance) throws SQLException {
+        if (instance.getId() < 0) return false;
         setStringOrNull(update, 1, instance.getLastname());
         setStringOrNull(update, 2, instance.getGivenname());
         setStringOrNull(update, 3, instance.getMail());