Run Time error

Hi,

when iam executing this program iam getting Run time error.

REPORT ZAC_TAB25.

DATA:

W_RESULT TYPE C.

PARAMETERS:

P_NUM1 TYPE I,

P_NUM2 TYPE I,

P_OPER TYPE C.

CALL FUNCTION 'ZAC_CALCULATE'

EXPORTING

IM_NUM1 = P_NUM1

IM_NUM2 = P_NUM2

IM_OPER = P_OPER

IMPORTING

EX_RESULT = W_RESULT.