C# if statement shorthand operators (? :) results in unreachable code

Why do I get this warning in C# with Visual Studio 2010?

"Unreachable expression code detected"

from the following code (DateTime.Now underlined in green squiggly):

public DateTime StartDate
{
  get
  {
    DateTime dt = (DateTime)ViewState["StartDate"];
    return ((dt == null) ? DateTime.Now : dt);
  }
}