invalid usage of aggregate function sum and type string. datatable

invalid usage of aggregate function sum and type string. datatable

i am trying to sum a column of datatable in vb.net but i am getting this error
invalid usage of aggregate function sum and type string.

Sample:

  dim sum  as integer = dtfilter.Compute("sum(Amount)", "1=1")

Screenshot:


Solution:

   Now I am using Custom function, I getting the result...

Example:

dim sum  as integer = CustomCompute(dtfilter, "Amount")

Private Function CustomCompute(ByVal Datatable As DataTable, ByVal SumColumn As String)
        Dim retval As Double = 0
        Try
            If Datatable IsNot Nothing Then
                For Each dr As DataRow In Datatable.Rows
                    retval = retval + Convert.ToInt32(dr(SumColumn))
                Next
                Return retval
            Else
                Return 0
            End If
        Catch ex As Exception
            Return 0
        End Try
End Function

Result: