diff --git a/payroll.py b/payroll.py
index 9b89c11..f9835ca 100644
--- a/payroll.py
+++ b/payroll.py
@@ -688,6 +688,8 @@ class Payroll(metaclass=PoolMeta):
                 unit_value, tax_base = self._compute_line_tax(line, wage)
                 if unit_value > 0:
                     tax_base = Decimal(str(round(tax_base, 2)))
+                else:
+                    tax_base = None
             else:
                 continue
             PayrollLine.write([line], {'unit_value': unit_value, 'tax_base': tax_base})