A quick question about the string syntax

quick question;
If I write somthing like X[d-1:0] then I’m reffering to the first string of X which is d long,
however what if d = 0?
is it defined? I’m essentially sending X[-1:0], what does this mean?

if d=0 the length of the string X is of 0 bits. the circuits need to be defined for an input with a positive length.