Dropdownlist selected value then move to next step

Hi 
What i am looking for is that user should select value from both DDL then when user click on the button he will be able to move to next step otherwise the label will show a message as bellow but the code its not working so i need a help to update it please

protected void nextpaybtn_Click(object sender, EventArgs e) { Session["plan"] = premplanddl.SelectedValue; Session["payment"] = paymethDDL.SelectedValue; if (premplanddl.SelectedValue == null) { Label14.Text = "Please select your plan"; } if (paymethDDL.SelectedValue == null) { Label14.Text = "Please select your payment method"; } MultiView1.ActiveViewIndex = 7; }