How to repro?

15:47:29 <Dereckson> .tcl registry incr quux 5
15:47:29 <Daeghrefn> Tcl error: can't read "term": no such variable

Stacktrace

can't read "term": no such variable
    while executing
"registry set $key $term"
    (procedure "registry" line 29)
    invoked from within
"registry incr quux 5"

Issue

The procedure registry contains code to put 1 as default increment value, but not to use the parameter.

if {$value == ""} {set term 1}

That should become:

if {$value == ""} {set term 1} {set term $value}

An error should be triggered if value isn't numeric.