passing shared variables from sub report

in my sub report

whileprintingrecords;

SHARED Numbervar  unitcost := {PO.UNITCOST};

in my main report

whileprintingrecords;

SHARED Numbervar  unitcost;

unitcost

the value return start with 0. what did i do wrong? I need then to match

              main      variable from sub report

item 1    50.00     00.00

item 2    60.00     50.00

item 3    70.00     60.00

item 4    80.00     70.00