jemalloc_arena.c:821: Failed assertion: "nstime_compare(&decay->epoc h, &time) <= 0")

I'm trying to stabilize my new-ish Ryzen package build box.  I've run
into this error a couple of times:
  (<jemalloc>: jemalloc_arena.c:821: Failed assertion: "nstime_compare(&decay->epoc h, &time) <= 0")
most recently today when building llvm40. It seems to happen somewhat
randomly.  I don't remember seeing it before the jemalloc 5.0.0 import.

What does it mean?  Any ideas on how I might mitigate the problem?