diff --git a/facho/fe/form/__init__.py b/facho/fe/form/__init__.py
index 93d55e8..617fba1 100644
--- a/facho/fe/form/__init__.py
+++ b/facho/fe/form/__init__.py
@@ -389,7 +389,7 @@ class InvoiceLine:
     # ya que al reportar los totales es sobre
     # la factura y el percent es unico por type_code
     # de subtotal
-    tax: TaxTotal
+    tax: typing.Optional[TaxTotal]
 
     @property
     def total_amount(self):
@@ -419,6 +419,9 @@ class InvoiceLine:
         if not isinstance(self.quantity, Quantity):
             raise ValueError("quantity must be Amount")
 
+        if self.tax is None:
+            self.tax = TaxTotalOmit()
+
 @dataclass
 class LegalMonetaryTotal:
     line_extension_amount: Amount = Amount(0.0)