diff --git a/addons/website_event_sale/models/sale_order.py b/addons/website_event_sale/models/sale_order.py
index d092b751366..296738c6925 100644
--- a/addons/website_event_sale/models/sale_order.py
+++ b/addons/website_event_sale/models/sale_order.py
@@ -53,6 +53,17 @@ class SaleOrder(models.Model):
     def _cart_update(self, product_id=None, line_id=None, add_qty=0, set_qty=0, **kwargs):
         OrderLine = self.env['sale.order.line']
 
+        try:
+            if add_qty:
+                add_qty = float(add_qty)
+        except ValueError:
+            add_qty = 1
+        try:
+            if set_qty:
+                set_qty = float(set_qty)
+        except ValueError:
+            set_qty = 0
+
         if line_id:
             line = OrderLine.browse(line_id)
             ticket = line.event_ticket_id