Matplotlib 2 mathtext: Glyph errors in tick labels

I've observed errors when rendering math in matplotlib 2.0.2, when using the default mathtext as opposed to the LaTeX math rendering engine. It seems that some glyphs (in my case the minus and the multiplication sign) is not recognized by mathtext. What makes it really weird is that the error only occurs when these particular glyphs appear in tick labels. When I deliberately type some mathy expression into e.g. the figure title, it works fine.

Consider the below example and the resultant image:

import matplotlib
import matplotlib.pyplot as plt

# Customize matplotlib
matplotlib.rcParams.update({# Use mathtext, not LaTeX
                            'text.usetex': False,
                            # Use the Computer modern font
                            'font.family': 'serif',
                            'font.serif': 'cmr10',
                            'mathtext.fontset': 'cm',
                            })

# Plot
plt.semilogy([-0.03, 0.05], [0.3, 0.05])
plt.title(r'$-6\times 10^{-2}$')
plt.savefig('test.png')

test.png

As seen on the image, multiplication and some minus signs in the tick labels have been replaced with other characters. If I use LaTeX (by setting 'text.usetex' to True), everything renders nicely. Why does this happen, and more importantly, how can I fix it without changing from mathtext to LaTeX?

Additional information

This is the warning which gets printed when running the example code:

mathtext.py:866: MathTextWarning: Font 'default' does not have a glyph for '\times' [U+d7]
  MathTextWarning)
mathtext.py:867: MathTextWarning: Substituting with a dummy symbol.
  warn("Substituting with a dummy symbol.", MathTextWarning)

Note that the minus signs appearing in exponents get rendered correctly. These also do not render probably if I leave out 'mathtext.fontset': 'cm', producing another, similar warning:

mathtext.py:866: MathTextWarning: Font 'default' does not have a glyph for '-' [U+2212]
  MathTextWarning)
mathtext.py:867: MathTextWarning: Substituting with a dummy symbol.
  warn("Substituting with a dummy symbol.", MathTextWarning)

Also, if I include 'axes.unicode_minus': False in the rcParams (and keep 'mathtext.fontset': 'cm'), all minus signs render properly, though the problem remains for the multiplication signs.

The multiplication sign error do not seem to be a problem on older versions of matplotlib (I've tested 1.5.1, 1.4.3 and 1.3.1). However, these matplotib's insist on only producing tick labels at 10⁻², 10⁻¹, 1, 10, 10² etc., and so no multiplication sign is ever needed.