In AD&D 2nd Edition, if a character, for example, has the Fishing Nonweapon Proficiency, the rules are clear about how to determine their success at fishing (`Each hour the character spends fishing, roll a proficiency check. If the roll is failed, no fish are caught that hour. Otherwise, a hook and line or a spear will land fish equal to the difference between the die roll and the character's Wisdom score.`)

But what if a character wants to attempt a task for which they don't have a nonweapon proficiency. Say a different character, that does not have the fishing proficiency wants to try to fish. How do you determine their success?

(To be clear, my question isn't about fishing, just using it as an example proficiency)

I've considered coming up with some kind of house rule (like roll the normal proficiency check roll with a -6 modifier) but I'm curious if there's an intended way to deal with this in the rules.