The issue here was that I was assuming IRC messages always have a leading colon at the start of the trailing message, like so:
:coulomb.oftc.net MODE #zzzz13 :+nt
As it turns out, the leading colon is optional based on whether it's necessary to disambiguate the message, so the server can send this instead:
:coulomb.oftc.net MODE #zzzz13 +nt
This release selectively parses the leading colon if it's present in the message, instead of requiring it to be present.