is method has problems when value is nil. workaround?

[Shadowblitz16]

There seems to be a problem with the design of the is method.
You can't do things like nil:is(SomeType)
This makes it pretty much useless for type checking as you have to guarantee that class variables are never nil.

I tried implementing it as a helper function with a little change...

function is(self, T)
  if self == T then
      return true
  else
      local mt = getmetatable(self)
      while mt do
      if mt == T then
          return true
      end
      mt = getmetatable(mt)
      end
  end
  return false
end

but it has the same problem.

Maybe there is a way to get around it?
This would also effect #20