mcover breaks null-safety

[byteit101]

Compiling with the macro enabled, I get this error: Null safety: Cannot cast nullable value to not nullable type

from this expression:

function myfunc(key:String):String {
    var tmp = mydict[key];
    if (tmp != null)
       return (tmp:String); // here is the line it complains about
    //...
}

The only way I can find to solve this is to disable null safety for the offending class, run mcover, then re-enable null safety