Conversion from type 'DBNull' to type 'String' is not valid.

Private Sub btnview_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnview.Click
    cn()
    str = InputBox("Enter Student Number", "Search by student number")
    sql = "select count (*) from table1 where stnum = '" & str & "'"
    com = New OleDbCommand(sql, con)
    If com.ExecuteScalar <> 0 Then
        sql = "select * from table1 where stnum='" & str & "'"
        com = New OleDbCommand(sql, con)
        dr = com.ExecuteReader
        dr.Read()
        txtstnum.Text = dr(0)
        txtstname.Text = dr(1)
        txtgender.Text = dr(2) 'my error is here
        txtage.Text = dr(3)
        txtcourse.Text = dr(4)
        txtyear.Text = dr(5)
        txtbdate.Text = dr(6)
        txtadd.Text = dr(7)
        txtcontact.Text = dr(8)
        PictureBox1.ImageLocation = "C:\Users\FRK\Documents\Visual Studio 2010\Projects\T_SIS\T_SIS\bin\Debug\studpics"
        dr.Close()
        con.Close()
    Else
        MessageBox.Show("Record not found", "Message")
        con.Close()
    End If
End Sub