diff --git a/htdocs/public/members/new.php b/htdocs/public/members/new.php
index d8c25bbebbb..e6b7a3e7a38 100644
--- a/htdocs/public/members/new.php
+++ b/htdocs/public/members/new.php
@@ -218,7 +218,7 @@ if (empty($reshook) && $action == 'add') {
 		$error++;
 		$errmsg .= $langs->trans("ErrorFieldRequired", $langs->transnoentitiesnoconv("Lastname"))."<br>\n";
 	}
-	if (GETPOST("firstname")) {
+	if (!GETPOST("firstname")) {
 		$error++;
 		$errmsg .= $langs->trans("ErrorFieldRequired", $langs->transnoentitiesnoconv("Firstname"))."<br>\n";
 	}