Why does a switch statement on an enum throw 'not comparable to type' error?

Question

While doing a Lynda tutorial on Typescript ( https://www.lynda.com/Visual-Studio-tutorials/TypeScript-types-part-2/543000/565613-4.html#tab ), I hit a snag. The sample code

Answer 1

Another possible reason could be if you check if the enum variable is not null but this also triggers if enumVal is 0. This only holds true if the enum has the default numeric values (so first item gets the value of 0)

if (!!enumVal) {
    switch (enumVal) {
         case EnumClass.First // the if clause automatically excludes the first item