$ docker exec -it tmate-websocket bin/tmate remote_console
Erlang/OTP 22 [erts-10.7.2.8] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1]

Interactive Elixir (1.9.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(tmate@tmate-ws.example.com)1> {:ok, result} = Tmate.MasterApi.get("/named_session_prefix", [], params: %{api_key: "tmk-mytoken"})
{:ok, "{\"prefix\":\"myusername/\"}"}
iex(tmate@tmate-ws.example.com)2> result["prefix"]
** (FunctionClauseError) no function clause matching in Access.get/3    
    
    The following arguments were given to Access.get/3:
    
        # 1
        "{\"prefix\":\"myusername/\"}"
    
        # 2
        "prefix"
    
        # 3
        nil
    
    Attempted function clauses (showing 5 out of 5):
    
        def get(-%module{} = container-, key, default)
        def get(map, key, default) when -is_map(map)-
        def get(list, key, default) when -is_list(list)- and -is_atom(key)-
        def get(list, key, _default) when -is_list(list)-
        def get(-nil-, _key, default)
    
    (elixir) lib/access.ex:265: Access.get/3
iex(tmate@tmate-ws.example.com)2> is_map(result)
false
iex(tmate@tmate-ws.example.com)7> is_binary(result)
true

diff --git a/lib/tmate/master_api.ex b/lib/tmate/master_api.ex
index acc5c33..6ee1425 100644
--- a/lib/tmate/master_api.ex
+++ b/lib/tmate/master_api.ex
@@ -29,7 +29,7 @@ defmodule Tmate.MasterApi do
   def get_named_session_prefix(api_key) do
     case get("/named_session_prefix", [], params: %{api_key: api_key}) do
       {:ok, result} ->
-        {:ok, result["prefix"]}
+        {:ok, Jason.decode!(result)["prefix"]}
       {:error, 404} ->
         {:error, :not_found}
       {:error, reason} ->