diff --git a/nur_src/b/ValueException.php b/nur_src/b/ValueException.php
index 5dc47df..4cba1cb 100644
--- a/nur_src/b/ValueException.php
+++ b/nur_src/b/ValueException.php
@@ -9,22 +9,22 @@ use nur\A;
  */
 class ValueException extends UserException {
   /** Retourner $value si elle n'est pas false. sinon lancer une exception */
-  static final function check_nf($value, $message=null) {
-    if ($value !== false) return $value;
+  static final function check_nf($value, $message=null, bool $required=true) {
+    if ($value !== false || !$required) return $value;
     if ($message === null) $message = "value must not be false";
     throw new self($message);
   }
 
   /** Retourner $value si elle n'est pas null. sinon lancer une exception */
-  static final function check_nn($value, $message=null) {
-    if ($value !== null) return $value;
+  static final function check_nn($value, $message=null, bool $required=true) {
+    if ($value !== null || !$required) return $value;
     if ($message === null) $message = "value must not be null";
     throw new self($message);
   }
 
   /** Retourner $value si elle n'est ni null ni false. sinon lancer une exception */
-  static final function check_nz($value, $message=null) {
-    if ($value !== false && $value !== null) return $value;
+  static final function check_nz($value, $message=null, bool $required=true) {
+    if (($value !== false && $value !== null) || !$required) return $value;
     if ($message === null) $message = "value must not be false nor null";
     throw new ValueException($message);
   }