Question: How to define a tenso of rank zero?

Download HowToDefineTensorRankZero.mw

Using the latest Physics package, I try define a tensor of rank zero, but an error accurs:

> Define(Theta = varepsilon[mu, ~mu](X));
Error, (in Physics:-Define) expected left-hand-side of tensor definition as a tensor with non-numerical indices, received: Theta

The same error occurs when I try:
> Define(Theta[] = varepsilon[mu, ~mu](X));
Error, (in Physics:-Define) expected left-hand-side of tensor definition as a tensor with non-numerical indices, received: Theta[]

So my question is: how can I define a tensor of rank zero? 
Hope someone can help me out. 

Regards, Wouter

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