Question

I was reading Lehninger and stumbled upon the following:
By convention, if the links between two DNA strands are arranged so that the strands are interwound in a right-handed helix, the linking number is defined as positive (+); for strands interwound in a left-handed helix, the linking number is negative (-). Negative linking numbers are, for all practical purposes, not encountered in DNA.
By linking number they must be talking about twists. My question is since we have Z-DNA whose strands are in left-handed helix can't its Twist be negative?