Anyone else getting 'empty comment' spam after 4.7.2 upgrade?

Is anyone else getting spammed with empty comments? ie both subject and body are blank.

I've started getting them right after completing the 4.7.2 upgrade. I'm assuming they are from spammers as they arrive in groups of two or three from IP addresses all around the world.

Any ideas what changes in 4.7.2 could allow this? I was not getting the problem with 4.7.0.

If I visit the site and try and enter a blank comment as an anonymous user it will give me an error message. So not sure how the spammers bypass that?

I've also installed captcha. Again, it works for humans, but the spam still gets around it.

Any ideas?

Regards, MrB