diff --git a/htdocs/comm/propal.php b/htdocs/comm/propal.php
index 9b331027159..e5b203a9d8d 100644
--- a/htdocs/comm/propal.php
+++ b/htdocs/comm/propal.php
@@ -641,7 +641,7 @@ else if ($action == "addline" && $user->rights->propal->creer)
 		setEventMessage($langs->trans("ErrorFieldRequired",$langs->transnoentitiesnoconv("Type")), 'errors');
 		$error++;
 	}
-	if ((empty($idprod) || GETPOST('usenewaddlineform')) && (!($price_ht >= 0) || $price_ht == ''))	// Unit price can be 0 but not ''
+	if ((empty($idprod) || GETPOST('usenewaddlineform')) && (!($price_ht != 0) || $price_ht == ''))	// Unit price can be 0 but not ''. Also price can be negative for proposal.
 	{
 		setEventMessage($langs->trans("ErrorFieldRequired",$langs->transnoentitiesnoconv("UnitPriceHT")), 'errors');
 		$error++;