No, the problem is in how you:
A) Definite it clearly enough to include one and exclude the other
B) Make it sufficiently in the interest of all countries to want to do so.
It's B that's really going to be the hard part. Weapons generally don't get banned because they're morally horrifying or repugnant, they get banned because countries come to the conclusion that using them really just isn't worth it, and that we'd be better off agreeing to not do so, EVEN IF SOMEONE ELSE DECIDES TO VIOLATE THAT.
Consider Chemical Weapons, something that despite their widespread proliferation, generally was never employed in warfare with the exception of WWI and the Iran-Iraq war. In both of the wars CW were employed in, they were generally indecisive in the outcome of the war. They're also rather prone to affecting your own troops, and one can employ countermeasures without employing the weapons yourself. In short, they're not worth the trouble, expense, and time to maintain them.
Nuclear weapons, on the other hand, are clearly quite potent, and useful as a tool of state preservation (or at least perceived to be). Despite how horrible these weapons are, and how incomprehensibly awful it would/will be if they ever get used again, the best we've managed to do is reduce the number of them (because again, they're expensive, and we can spend less while still maintaining the same effect).
Now consider another weapon - Land Mines. These are banned by the Ottawa Treaty, which has an impressive number of signatories... but notably lacks the participation of the USA, China, or Russia, or of certain countries with significantly hostile borders such as India/Pakistan and North and South Korea. Basically, anyone who still thinks they might need these weapons.
So where does that leave highly autonomous combat systems? Well, my take is that war has long been a factor of three things - population, technology, and production capacity. We're talking about something that takes one of those almost entirely out of the equation. Does that seem like a potentially powerful thing, or something that no one would have any interest in ever using?