[hl] Fix weird compiler error

[Apprentice-Alchemist]

Closes #11689

[Simn]

@ncannasse Could you check/merge this?

[Simn]

I checked the documentation for Marshal.Closures and this seems good to me:

If flags contains Marshal.Closures, functional values will be marshaled as a the position in the code of the program together with the values corresponding to the free variables captured in the closure. In this case, the output of marshaling can only be read back in processes that run exactly the same program, with exactly the same compiled code.