"Invalid triangles" when isosurface value is not 0.0

[Ickaser]

Something goes wrong with the algorithm when I use a nonzero isosurface value.

Here's my MWE:

using MarchingCubes
xi = [1.0, 2, 3, 4, 5]
dat = [(x-3)^2 + (y-3)^2 + (z-3)^2 for x in xi, y in xi, z in xi]
datsub = dat .- 5.0

m1 = MC(dat)
march(m1, 5.0)

m2 = MC(datsub)
march(m2)

march(m1, 5,0) gives me a lot of warnings about invalid triangles:

┌ Warning: Invalid triangle 6
└ @ MarchingCubes C:\Users\iwheeler\.julia\packages\MarchingCubes\V0hg7\src\MarchingCubes.jl:557
┌ Warning: Invalid triangle 12
└ @ MarchingCubes C:\Users\iwheeler\.julia\packages\MarchingCubes\V0hg7\src\MarchingCubes.jl:557

on up to triangle 128, with many repeats. On the other hand,
march(m2) runs just fine, and when I examine its output with PlyIO and Paraview, it gives me the sphere I expect.