Imagebutton.ImageUrl won't let read

[doll]

    private void changeImageUrl()
    {
        string an= Request.QueryString["an"];

        if (an == "Middag")
        {
            if (imgBtnMelk.ImageUrl == "~/Images/bw_middag.jpg")
            {
                lblTest.Text = "Boo!"; // why won't it be read?
            }
        }

I've tested the label everywhere and figured out the wrong code must be:
if (imgBtnMelk.ImageUrl == "~/Images/bw_middag.jpg")

Does anyone have another suggestion for it?

(If i let lblTest.Text show imgBtnMelk.ImageUrl, it will show ~/Images/bw_middag.jpg.)