Recursion Issue

Hi,

I am attempting to solve a problem in Exercism “Protein Translation” and implemented the following code. I want the split(tail, final_list) to recursively add the 3 char to final_list. What am I doing wrong and how can I improve the code.

defmodule PT do

  def rna(string) do
    split(string, [])
  end

  def split(string, final_list) when string == "", do: final_list

  def split(string, final_list) do
    splitting = String.split_at(string, 3)
    [head | tail] = Tuple.to_list(splitting)

    final_list ++ [head]
    split(tail, final_list)
  end
end

i am getting this error:

iex(21)> PT.rna("Owaissdui")
** (FunctionClauseError) no function clause matching in String.Unicode.next_grapheme_size/1    
    
    The following arguments were given to String.Unicode.next_grapheme_size/1:
    
        # 1
        ["issdui"]
    
    Attempted function clauses (showing 10 out of 19263):
    
        def next_grapheme_size(<<13::integer(), 10::integer(), rest::binary()>>)
        def next_grapheme_size(<<"\r"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"\n"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"󠇰"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"󠇱"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"󠇲"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"󠇳"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"󠇴"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"󠇵"::binary(), rest::binary()>>)
        def next_grapheme_size(<<"󠇶"::binary(), rest::binary()>>)
        ...
        (19253 clauses not shown)

Thanks