diff --git a/invoice.py b/invoice.py
index 39b01d48b2..a67f7e3641 100644
--- a/invoice.py
+++ b/invoice.py
@@ -1782,7 +1782,7 @@ class InvoiceLine(sequence_ordered(), ModelSQL, ModelView, TaxableMixin):
                 self.description = Product(self.product.id).rec_name
 
         category = self.product.default_uom.category
-        if not self.unit or self.unit not in category.uoms:
+        if not self.unit or self.unit.category != category:
             self.unit = self.product.default_uom.id
             self.unit_digits = self.product.default_uom.digits