AX 2012 workflow conditional decision

Hello,

I am configuring a workflow conditional decision, that will fire when an invoices salesTotal()(calculated field) is over 1000. But when I insert the condition (salesTable.salesTotal > 1000), it gives me an error

"A currency code must be specified for an amount field."

I cannot insert a currency code for that field. I am using a custom EDT for that field, that is extended from LineAmount. When I insert another field like salesLine.lineamount in the expression, I am automatically given an option to choose a currency for the value.

Could anyone explain to me, how I could tell Axapta that this EDT is a currency, or maybe something else is wrong???

Thank, Karl