Guard `in` list doesn't work in function's parameters

Hello , I have 3 fn which create and check list, but it does not work for me .

for example :

list creator :

defp string_to_array(string) when is_binary(string) do
        String.split(string, [" ", ","])
end

and check fn :

defp merge_db_plug_category(query, group_acl) when group_acl in ["admin"] do
		IO.puts "1"
		IO.inspect group_acl
end

defp merge_db_plug_category(query, group_acl)  do
		IO.puts "2"
		IO.inspect group_acl
end

I get ["admin"] list when I send and inspect group_acl, but always merge_db_plug_category(query, group_acl) works , why doesn’t when group_acl in ["admin"] work for me ?

how can I check list in function parameters ?