Has anyone seen this error with Hummingbird Caching?

Has anyone seen this error within their WP dashboard?

"The first argument should be either a string or an integer in /home/wpmudev/public_html/network/wp-includes/cache.php on line 700"

I'm running a multisite with Hummingbird as my only caching plugin. I noticed that this error pops when I add a new user. I first saw it when I added a new site for a customer. But again it's not so much the adding a new site...This error shows when I add a new user.

Not sure if it's a Hummingbird issue? It's kind of annoying having to look at it everyday. My customers are also asking what this is all about...

Any advice on how to fix will be appreciated. Thanks!