diff --git a/admin/sync/exercice_keys.go b/admin/sync/exercice_keys.go
index 77812442..10030fac 100644
--- a/admin/sync/exercice_keys.go
+++ b/admin/sync/exercice_keys.go
@@ -169,6 +169,10 @@ func buildKeyFlag(exercice fic.Exercice, flag ExerciceFlag, flagline int, defaul
 				Value: val,
 			})
 
+			if val == "true" || val == "false" {
+				errs = append(errs, fmt.Sprintf("%q: flag #%d: value can't be %q, this is not a MCQ, the value has to be meaningful. The value is shown to players as response identifier.", path.Base(exercice.Path), flagline), val)
+			}
+
 			if val == raw || (!flag.CaseSensitive && val == strings.ToLower(raw)) {
 				hasOne = true
 			}