Period in infopath formula clearing the field

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I have this formula in one of the fields in a repeating table. If chk checkbox is false, it's supposed to leave the value as it is which I typed in the field. When the form is loaded, the field value shows 0. When I enter new value,its is being cleared and become zero again.

(../my:TotalCost | .)[(../my:chk = string(true())) + 1]

What I have tried:

Tried everything. But, can't find a solution.