action

export var startToggleTodo  = (id, completed) => {
  return (dispatch, getState) => {
    /*access item we need to change*/
    var todoRef = firebaseRef.child(`todos/${id}`);

    /*generate Updates*/
    var updates = {
      completed,
      completedAt: completed ? moment.unix() : null
    };

    /*start updating*/
    todoRef.update(updates).then(() => {
      dispatch(updateTodo(id, updates));
    });

  };
};

error

firebase.js?678b:426 Uncaught Error: Firebase.update failed: First argument contains NaN in property 'todos.-KQ_BiN5I0nel2NrBnxl.completedAt._i'22