diff --git a/product/ERP5/Document/TradeModelLine.py b/product/ERP5/Document/TradeModelLine.py
index 0123fa00c012de5426509436367e78cd706dd0a4..03207428fe871c3382c90a6100be5c9e53d728c9 100644
--- a/product/ERP5/Document/TradeModelLine.py
+++ b/product/ERP5/Document/TradeModelLine.py
@@ -83,8 +83,8 @@ class TradeModelLine(MappedValue, XMLMatrix, Amount, AmountGeneratorMixin):
     result = self._baseGetMappedValuePropertyList()
     if result:
       return result
-    if self._baseGetQuantity(): # If quantity is defined, then tax works as transformed resource
-      return ('quantity', 'price', )
+    if self._baseGetQuantity() is not None: # If quantity is defined, then tax works as transformed resource
+      return ('quantity', 'price', 'step')
     # Else tax provides only a ratio on amount
     return ('price', 'efficiency')