diff --git a/htdocs/langs/en_US/bills.lang b/htdocs/langs/en_US/bills.lang
index 8e922506b23..ddef01b17c1 100644
--- a/htdocs/langs/en_US/bills.lang
+++ b/htdocs/langs/en_US/bills.lang
@@ -151,7 +151,7 @@ ErrorBillNotFound=Invoice %s does not exist
 ErrorInvoiceAlreadyReplaced=Error, you tried to validate an invoice to replace invoice %s. But this one has already been replaced by invoice %s.
 ErrorDiscountAlreadyUsed=Error, discount already used
 ErrorInvoiceAvoirMustBeNegative=Error, correct invoice must have a negative amount
-ErrorInvoiceOfThisTypeMustBePositive=Error, this type of invoice must have a positive amount
+ErrorInvoiceOfThisTypeMustBePositive=Error, this type of invoice must have an amount excluding tax positive (or null)
 ErrorCantCancelIfReplacementInvoiceNotValidated=Error, can't cancel an invoice that has been replaced by another invoice that is still in draft status
 ErrorThisPartOrAnotherIsAlreadyUsedSoDiscountSerieCantBeRemoved=This part or another is already used so discount series cannot be removed.
 BillFrom=From