diff --git a/SAS/TMSS/backend/src/tmss/tmssapp/migrations/0002_populate.py b/SAS/TMSS/backend/src/tmss/tmssapp/migrations/0002_populate.py
index 92baffd4c15a8c025d234eeffed61ae9f443fabf..cc01b09d6659e81e82e1001073ba00d044e1aa95 100644
--- a/SAS/TMSS/backend/src/tmss/tmssapp/migrations/0002_populate.py
+++ b/SAS/TMSS/backend/src/tmss/tmssapp/migrations/0002_populate.py
@@ -14,9 +14,24 @@ class Migration(migrations.Migration):
         ('tmssapp', '0001_initial'),
     ]
 
-    # Start SubTask id with 2 000 000 to avoid overlap with 'old' (test/production) OTDB
-    operations = [ migrations.RunSQL('ALTER SEQUENCE tmssapp_SubTask_id_seq RESTART WITH 2000000;'),
-                   migrations.RunPython(populate_choices),
+
+    operations = [ migrations.RunSQL('ALTER SEQUENCE tmssapp_SubTask_id_seq RESTART WITH 2000000;'), # Start SubTask id with 2 000 000 to avoid overlap with 'old' (test/production) OTDB
+                   migrations.RunPython(populate_choices), # poppulates the "fixed" enums, with amongst others the subtask_states which are used next in the trigger
+                   migrations.RunSQL("CREATE OR REPLACE FUNCTION tmssapp_check_subtask_state_transition()                            \
+                                     RETURNS trigger AS                                                                              \
+                                     $BODY$                                                                                          \
+                                     BEGIN                                                                                           \
+                                       IF OLD.state_id='defining' AND NEW.state_id != 'defined' THEN                                 \
+                                         RAISE EXCEPTION 'ILLEGAL SUBTASK STATE TRANSITION FROM % TO %', OLD.state_id, NEW.state_id; \
+                                       END IF;                                                                                       \
+                                     RETURN NEW;                                                                                     \
+                                     END;                                                                                            \
+                                     $BODY$                                                                                          \
+                                     LANGUAGE plpgsql VOLATILE;                                                                      \
+                                     DROP TRIGGER IF EXISTS tmssapp_trigger_on_check_subtask_state_transition ON tmssapp_SubTask ;   \
+                                     CREATE TRIGGER tmssapp_trigger_on_check_subtask_state_transition                                \
+                                     AFTER UPDATE ON tmssapp_SubTask                                                                 \
+                                     FOR EACH ROW EXECUTE PROCEDURE tmssapp_check_subtask_state_transition();"),
                    migrations.RunPython(populate_settings),
                    migrations.RunPython(populate_misc),
                    migrations.RunPython(populate_resources),