list comprehension fails, while map works

I'm calling erlang function, that calls reia back in the follwing way:

A = ["app","aaa","qqq"],
'Reia_module':f(A),

Reia_module:

module Reia_module
  def f(a)

# this works fine:
    b = a.map { |x| x.to_string() } # convert each element to string, since that's a list of lists

# this one fails:
    c = [x.to_string() | x in a]

The exception is:

Reason: {function_clause,[{reia_list,to_erl,[["app","aaa","qqq"]]},