I'm not sure this is relevant but [the manual says](https://nim-lang.org/docs/manual.html#parallel-spawn-spawn-statement) :
> Due to technical limitations not every type T is possible in a data flow > variable: **T has to be of the type ref, string, seq or of a type that > doesn't contain a type that is garbage collected**. This restriction is not > hard to work-around in practice. So, it might be because your tree contains a _ref_ (which is GC'd) and it is not itself a _ref_
