diff --git a/htdocs/mrp/class/mo.class.php b/htdocs/mrp/class/mo.class.php
index 7745d05d71a..97075ddd440 100644
--- a/htdocs/mrp/class/mo.class.php
+++ b/htdocs/mrp/class/mo.class.php
@@ -638,8 +638,8 @@ class Mo extends CommonObject
 
 		$oldQty = $this->oldQty;
 		$newQty = $this->qty;
-		if($newQty != $oldQty) {
-			$sql = 'SELECT rowid FROM ' . MAIN_DB_PREFIX . 'mrp_production WHERE fk_mo = ' . $this->id;
+		if($newQty != $oldQty && !empty($this->oldQty)) {
+			$sql = 'SELECT rowid FROM ' . MAIN_DB_PREFIX . 'mrp_production WHERE fk_mo = ' . (int) $this->id;
 			$resql = $this->db->query($sql);
 			if($resql) {
 				while ($obj = $this->db->fetch_object($resql)) {