That's true. I just want to add that this is not the only case where we don't (or prefer not to) use an article. Lundgren8 wrote something interesting about this here: https://www.duolingo.com/comment/5824774
raster is the plural of the noun en rast 'a break' so that doesn't work at all.
There's a verb rasta which would be rastar in the present tense, but that's used more for things like walking your dog (rasta hunden).