Equation example gives error

[JennaVergeynst]

Hi,

I installed stl_tools, but when I try the equation example:

from pylab import imread
from stl_tools import numpy2stl, text2png, text2array

text = ("$\oint_{\Gamma} (A\, dx + B\, dy) = \iint_{U} \left(\\frac{\partial "
        "B}{\partial x} - \\frac{\partial A}{\partial y}\\right)\ dxdy$ \n\n "
        "$\\frac{\partial \\rho}{\partial t} + \\frac{\partial}{\partial x_j}"
        "\left[ \\rho u_j \\right] = 0$")
text2png(text, "Greens-Theorem_Navier-Stokes", fontsize=50) #save png

A = imread("Greens-Theorem_Navier-Stokes.png") # read from rendered png
A = A.mean(axis=2) #grayscale projection
A = gaussian_filter(A.max() - A, 1.)

numpy2stl(A, "Greens-Theorem_Navier-Stokes.stl", scale=0.2, mask_val = 5.)

This gives following error:

ValueError                                Traceback (most recent call last)
<ipython-input-78-d36112c9354d> in <module>()
     12 A = gaussian_filter(A.max() - A, 1.)
     13 
---> 14 numpy2stl(A, "Greens-Theorem_Navier-Stokes.stl", scale=0.2, mask_val = 5.)

/Applications/miniconda3/envs/work/lib/python3.5/site-packages/stl_tools/numpy2stl.py in numpy2stl(A, fn, scale, mask_val, ascii, max_width, max_depth, max_height, solid, min_thickness_percent, force_python)
    187             facets = np.concatenate([facets, bottom])
    188 
--> 189     xsize = facets[:, 3::3].ptp()
    190     if xsize > max_width:
    191         facets = facets * float(max_width) / xsize

ValueError: zero-size array to reduction operation maximum which has no identity

This is the same error as when I use it for my own numpy arrays. Any idea?