Meaning Externalism

It is a simple matter to conceive of myself as being a brain in a vat. I know what that is, and I know what it would mean for me to be one.

You chuckle. And mention Putnam.

But is there any reason why we shouldn't construe Putnam's argument as constituting a reductio ad absurdum of meaning externalism? I mean, any reason other than the fact that Putnam didn't mean for it to be, despite the fact that it is.