diff --git a/modules/purchase_trade/purchase.py b/modules/purchase_trade/purchase.py
index 493f7ce..81a355a 100755
--- a/modules/purchase_trade/purchase.py
+++ b/modules/purchase_trade/purchase.py
@@ -723,9 +723,10 @@ class AssayElement(ModelSQL, ModelView):
     default_unit = fields.Many2One('product.uom', "Default Unit")
 
     type = fields.Selection([
+        (None, ''),
         ('metal', 'Metal'),
         ('impurity', 'Impurity'),
-    ])
+    ], "Type")
 
 class QualityAnalysis(ModelSQL,ModelView):
     "Quality Analysis"