Dear Experts
I use following codes to retrieve data from table, but get error

Conversion from type 'DBNull' to type 'String' is not valid.
Code:
str = "Select * from gpass where vou_no= " & Val(Me.txtGat.Text) & " And Date =   '" & Me.txtDat.Text & "'"
        dt = GetTable(str)

        If (dt.Rows.Count > 0) Then


            DataGridView1.Rows.Clear()

            For i = 0 To dt.Rows.Count - 1

                Me.DataGridView1.Rows.Add()
                Me.DataGridView1.Rows(i).Cells(1).Value = IIf(IsDBNull(dt.Rows(i)("tittle")), " ", Trim(dt.Rows(i)("tittle")))
            Next
        End If
End If
Tittle column of gpass is as

Column------datatype
Tittle--------char(100)

And data in column is as
Null
Null
Null


Please help