It uses try - catch, which is not in NaviServer 4.99.8 ...
"try" has nothing to do with NaviServer, it is part of Tcl 8.6 (released in 2013)
If for whatever reason you have to use Tcl 8.5, you can use the "try" package in tcllib for Tcl 8.5 upward compatibility.
all the best -g
