UpdateUserData return json error

I'm getting an error when trying to run this code in Unity 2018.3 : The detailed error message says "The value for key test_key was not valid JSON."

PlayFabClientAPI.UpdateUserData(new UpdateUserDataRequest() {
            Data = new Dictionary<string, string>() {
                {"key", "value"}
            },
            Permission = UserDataPermission.Public
        }, 
        result => Debug.Log("Successfully updated user data"),
        error => {
            Debug.Log("UpdateUserData error");
        });