Crystal report formula field returning error

The below formula return this error message:

Invalid Index. Exception from HRESULT: 0x8002000B (DISP_E_BADINDEX)

What I have tried:

I have created this crystal report formula in a formula field:

whileprintingrecords;
shared numbervar doBal:={?@obDebit};
shared numbervar dValue:=sum({command.debit});
shared numbervar dcBal:={?@cbDebit};
doBal + dValue + dcBal