diff --git a/htdocs/public/payment/paymentok.php b/htdocs/public/payment/paymentok.php
index 6cfc2e4e81b..bd12e9be48c 100644
--- a/htdocs/public/payment/paymentok.php
+++ b/htdocs/public/payment/paymentok.php
@@ -121,6 +121,7 @@ if (preg_match('/PM=([^\.]+)/', $FULLTAG, $reg)) {
 	$paymentmethod = $reg[1];
 }
 if (empty($paymentmethod)) {
+	dol_syslog("***** paymentok.php was called with a non valid parameter FULLTAG=".$FULLTAG, LOG_DEBUG, 0, '_payment');
 	dol_print_error(null, 'The callback url does not contains a parameter fulltag that should help us to find the payment method used');
 	exit;
 }