Why data source cannot be bounded to the textbox?

Hello,

I have a data source (stored procedure) for the table and I double checked and confirmed the sp returns the needed fields to be bounded to the table, why I still got errors like the following? How do I fix it? Thanks.

Error    1    The Value expression for the textbox ‘textbox38’ refers to the field ‘Workflow’.  Report item expressions can only refer to fields within the current data set scope or, if inside an aggregate, the specified data set scope.    C:\RXie\wwwroot\GSH\Reports\Report02WithParam.rdlc   1