I found this thread while searching for the assert failure, and but am experiencing it with a strictly blueprint set up (prototyping in BP, blah blah).
Thanks to 's comment above, I was able to realize that only the instanced mesh set as the ROOT component would work to an arbitrary number of instances; unchecking AutoWeld on the StaticMesh properties allowed me to use any of the components.
Again, I’m not doing any of this in the code right now, but it fixed the issue in BP. Whether this fix creates further issues is to be seen!