Thank you very much!
I had already gone through the Types and More about types pages in the docs, but it’s a lot to digest, so I missed that particular aspect. The fact that when testing with tuples it hadn’t tripped didn’t help indeed 
I’ve gone with AbstractVector{Tuple{M,String}} where M <: AbstractMatrix{<:Number}, but now I’ve got a related problem for “diagonal types”, if my understanding is correct. I’ll open a new topic for it. Thanks again.