"what i lack"

Saw this in a text:

"What I lack is words..."

Sounded immediately odd to my ear. Although I realize the predicate is ambiguous here, i.e.:

"THAT WHICH I LACK is X "

However, this sounds natural: "Words ARE what I lack." And it shouldn't be the case that word order affects construction.

Is this an issue of an ambiguous predicate?

This seems to be the case with "what I lack" and "what I need"...that type of phrase...

Any insights??