Null pointer exception for date

I am trying to run a script include that will skip the weekend. I keep receiving an error on getDayOfWeek()

java.lang.NullPointerException

function checkDate(x) {
   var gdt = new GlideDateTime(x);
   var day = gdt.getDayOfWeek();
   if (day == 1) {
   return gdt < gs.daysAgo(3);
   } else {
   return gdt < gs.daysAgo(1);
   }
}