can't launch wwiv-update on a subst'ed drive

[wwiv]

Example:

subst Z: C:\users\rushfan
Z:\bbs>wwiv-update.exe
The system cannot find the file Z:\bbs\wwiv-update.exe.

yet if I go to C:\users\rushfan\bbs it works.

C:
cd \users\rushfan
C:\users\rushfan>wwiv-update.exe

(it works here)