lolo-sources doesn't currently provide virtual/linux-sources, which confuses some ebuilds (like nvidia-kernel) when it's the only kernel tree installed. Reproducible: Always Steps to Reproduce:
they do ... they inherit the kernel.eclass which sets up the virtual
The bug must be in Portage itself. For some reason, it wasn't recognizing that my installed lolo-sources were providing linux-sources. Unmerging all my kernel trees and re-emerging lolo-sources got Portage to recognize that they do provide linux-sources, so the problem was probably a holdover from a previous version of Portage.