This is not a 'bug' as such, but just illogical behaviour by mirc. $addtok and $deltok (and perhaps other token identifers that I haven't tested) return nothing when a null token is used.

For example,
$addtok(a b c d,$network,32) = a b c d AustNet
$addtok(a b c d,$lakjaefh,32) = returns nothing

This should return "a b c d" since it's generally what would be expected.