Add a warning when using matrix instead of array

[talolard]

I just spent a few hours debugging this. I tried to play with this using a tfidf vectorizer which returns a sparse matrix. That didn't work with cryptic errors. I eventually tried todense but that didn't help.

After a long time, I thought, maybe toarray and that solved it. No idea why

[cosmic-cortex]

Hi!

Can you give a code snippet with the error log? I am not exactly sure what you refer to, but I would be happy to take a closer look.