# Why is GRASS not thinning a terraflow raster properly?

I want to convert a raster map encoding directional flows (the output of r.terraflow) to a vector map. I'm using r.thin to process the output. It terminates successfully with 0 pixels removed during the last iteration. Yet when I try to vectorise the result with r.to.vect and feature=line, I get the following error message:

Extracting lines...
ERROR: Raster map is not thinned properly.

I have tried rerunning r.thin on the already thinned output, with the same result. I did not find anything in the documentation of either r.thin or r.to.vect to suggest what might be the problem here or how to fix it.