Snail formatter problem with non-Base64 characters

When running juliaformatter through the snail interface (C-c j f b) on a file with a character outside of base64 (specifically \psi, I get the following message:
format: Multibyte character in data for base64 encoding

However, if I apply the formatter directly, using
using JuliaFormatter format(filename.jl)
then it works just fine.

Is there something that I’m missing?