Hi, this is due to the fact that zDir may contain spaces especially under windows which confuse the command processor to interpret it as multiple arguments and not a single one as with single or double quotes (http://libcox.net/arch.html)
That'd be my guess, at least, but, i guess, single quotes aren't valid characters for zDir.
(In the absence of this, and certainly in the absence of official docs/examples that do the right thing, that's a security vulnerability waiting to happen.)