crystalreport 2008 ,formula fields

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CrystalReports

Hello,

I wrote this code in formula fields.

shared numbervar bb;
if isnull({tbl1.sumbed}) and isnull({tbl1.sumbes})
then 0
else bb := bb + ({tbl1.sumbed}-{tbl1.sumbes});

I build tbl1 in DataSet. The fields in it same as fields in my query.

But I have this error: A Number,or currency fields,date,time is required.

Please someone help me.