When S_opmethod_stash() determines that the LHS is an unblessed reference, it generates an error message containing the method name. However, it was calling SvPVX() on the method name without ensuring that it's SvPOK(). The fix is straightforward, and is now in blead; see below.
That said, I'm wondering whether we should special-case undefined method names, so as to produce better error messages. If we want to do that before 5.26, we've got about two weeks left before the "user-visible changes" freeze.