crstal report . when converting total amount in to words

when converting total amount in to words  i am getting error anumber or currency is requied  . In this  i used dataset  and datatable  in asp.net

 

 formula for convertin total amt in to words 

 

Towords({@amtwords},0) +"and"+ ToWards({Round({@amtwords},2} - Int({@amtwords})) * 100,0)+" paise only ".

 

 

plz  help me