How to best deal with a cross site scripting threat

I have security audit scan that is flagging line 4 below
"The method DisplayCustomMessage() in clientSummary.cs sends unvalidated data to a web browser on line 4 which can result in the browser executing malicious code."

This appears to be a cross site scripting vulnerability. So how can I correct this code?

1.  private void DisplayCustomMessage(string msg)
2.  {
3.      lblMsg.CssClass = "pling";
4.      lblMsg.Text = msg
5.      lblMsg.Visible = true
6.
7.}