How to sum the running total field value and formula field value in crystal report?

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VB

CrystalReports

I want to add the running total field value and formula field value("Openbalnce"), the sum should be assigned to another formula field("Total").

In the formula editor of formula field("Total"),I have given as:

CDBL({RunningTotalValue}) + CDBL({FormulaField(Openbalnce)})


But it shows the error when i run the program as :

"The string is non-Numeric"

Please help me to sort out this issue.