Crystal reports : global subreport variable sum not transferring to parent main report

I am running crystal reports 11 and in the main report is a subreport which calculates the balance. In the subreport balance, I have the formula named balancetotal which manually sums the running total of the tarinvoice.balance field.

WhileReadingRecords;
global numbervar balancetotal;
balancetotal := balancetotal + {tarInvoice.Balance}

In the main parent report, i have a formula named balancetotal with this formula:

    global numbervar balancetotal;
    balancetotal;

is this incorrect?

thanks in advance