Unity 2018.1.0f2 Assertion failed on expression Errors

I keep getting about 28 of these errors at a time:

"Assertion failed on expression: '(srcData.GetChannelMask() & copyChannels) == copyChannels'"

I get this anytime I do anything to do with any prefab (select, search, save, apply, revert, etc.). Clicking on them tells me nothing about which objects are causing it and it just keeps popping up!

They're not causing any problems (crashes, build fails, or anything like that) so I mainly just ignore them, but I am curious about why it's happening. My guess is that this Unity version has an issue with 28 prefabs (or something that they're using, like a material(s)).

Any idea about how I can narrow down the cause(s) and get rid of these once and for all? I'm not getting this with any other Unity versions so it's specific for this one.

Thanks!