OK, then I'm confused.
What would the following return?
```
c = SpectralCube.read('file.fits')
c1 = c.with_spectral_unit(u.km/u.s)
c2 = c1.spectral_slab(5*u.km/s,10*u.km/s)
c2.spectral_axis.unit
```
I think it would return `m/s` but it should return `km/s`.  
.... 
oh ok you got it...