diff --git a/tax.py b/tax.py
index 03254a5133..f059b07c79 100644
--- a/tax.py
+++ b/tax.py
@@ -211,6 +211,7 @@ class Tax(OSV):
     percentage = fields.Numeric('Percentage', digits=(16, 8),
             states={
                 'invisible': "type != 'percentage'",
+                'required': "type == 'percentage'",
             }, help='In %')
     type = fields.Selection([
         ('percentage', 'Percentage'),