Tick labels do not appear for some parameters

Attempting to create a position-velocity plot, I end up with axis labels disappearing unexpectedly for some input parameters.

```
import wcsaxes
import pylab as pl
from astropy.io import fits
from astropy import wcs

fig1 = pl.figure(1)
pvhdu = fits.open('testhdu.fits')[0]
ax = wcsaxes.WCSAxesSubplot(fig1, 1, 1, 1, wcs=wcs.WCS(pvhdu.header))
ax.imshow(pvhdu.data)
fig1.add_axes(ax)
limits = wcs.WCS(pvhdu.header).wcs_world2pix([0,0],[35e3,80e3],0)[1]
print(limits) # should be 83, 128
ax.set_ylim(*limits)

ax.coords[1].set_ticks(spacing=4*u.km/u.s)
pl.draw(); pl.show()
# all is OK, but a bit weird

ax.coords[1].set_ticks(spacing=5*u.km/u.s)
pl.draw(); pl.show()
# EVERYTHING IS GONE!!!!! OH NOOOOOO!!!!!
```
OK (kinda):
![figure_1](https://cloud.githubusercontent.com/assets/143715/4124041/c5c8df86-32d4-11e4-8f2f-18bef420cee5.png)

Not OK:
![figure_2](https://cloud.githubusercontent.com/assets/143715/4124040/c5c7ef86-32d4-11e4-8f7b-10e050bbecf6.png)


Here's the FITS file:
https://www.dropbox.com/s/w2q2az4trsa5o8d/testhdu.fits?dl=0