Input String Is Not if Correct Formate

Imports System.Data
Imports System.Data.SqlClient

Partial Class _Default
    Inherits System.Web.UI.Page
    Dim strcon As String =  ConfigurationManager.ConnectionStrings("dbconnection").ConnectionString
    Dim con As New SqlConnection(strcon)
    Dim com As Integer = 0
    Dim suname As String

    //I got the Error that Input String Is Not if Correct Formate

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim cmd As New SqlCommand("Select * from tbl_abc where uname='" + Session("mail") + "'", con)
        Dim da As New SqlDataAdapter(cmd)
        Dim dt As New DataTable()
        da.Fill(dt)
        suname = dt.Rows(0)(3).ToString()

        Dim amount As Integer
        For amount = 1 To 8
            If suname = String.IsNullOrEmpty(suname) Then
                com += 20
            Else
                com = 0
            End If
        Next amount

        MsgBox(com)
    End Sub