Redux todo list advanced

I'm doing a todo list using redux and I want to add sub todo list for each todo, but I can't understand why my line for my reducer is not working (see above) Can you help me please ?

import { combineReducers } from 'redux'
import { ADD_TODO, TOGGLE_TODO, ADD_SUB_TODO} from '../constants/ActionTypes'
import _ from 'lodash'

const initialState = {
    todos : []
}

export function todo(state, action) {
    switch (action.type) {
        case  ADD_TODO:
            return {
                id: action.id,
                text: action.text,
                completed: false
            };
        default:
            return state;
    }
}


export function allTodo (state = initialState, action) {
    switch (action.type) {
        case  ADD_TODO:
            return {
                ...state,
                todos: [
                    ...state.todos,
                    {
                        id: action.id,
                        text: action.text,
                        completed: false,
                        subtodo:[]
                    }
                ]
            };
        case ADD_SUB_TODO:
            console.log("REDUCER")
            console.log(...state.todos)
            return {
                ...state,
        // THIS LINE DOES'NT WORK :
                ...state.todos[0].subtodo: [ ...state.todos[0].subtodo, {
                    id: action.id,
                    text: action.text
                }]
            };
        default:
            return state;
    }
};

export default combineReducers({
    allTodo
})

this line is not working :

...state.todos[0].subtodo: [ ...state.todos[0].subtodo, {
                        id: action.id,
                        text: action.text
                    }]

this is my sub todo object :

{
 id: action.id,
 text: action.text
}