[DotNetDevelopment] Why Convert dataTable in Crystal report + .net

[Musthaan]

I am trying to

 Dim ta As New ReprotSaleDataSetTableAdapters.DataTable1TableAdapter
 Dim dt As New ReprotSaleDataSet.DataTable1DataTable
 Dim rpt As New XrptSale8B
 ta.Fill(dt, TmpInvNo)
rpt.SetDataSource(dt)

                   But A error Occured.
and next try

Dim rpt As New XrptSale8B
 ta.Fill(dt, TmpInvNo)
rpt.SetDataSource(CType(dt, DataTable))

All is Fine

my doubt is , the dt is already a DataTable . Then Why Error occured
before converting  ?