How to add an array item inside array of objects of an object in react-redux toolkit?

I am trying to update the user object’s blockedUsers array, while keeping other data not changed. user object can have blockedUsers array or not, it is not certain. In slice I have:

    reducers: { CHANGE_USER: (state, action) => {
      state.user.push(action.payload)
    },
   }

And in component, I am trying to update this way:

dispatch(CHANGE_USER({blockedUsers: [...user?.blockedUsers, selectedUser] }))

where selectedUser is an object: {id: "xxxx", name: "yyyy"}. But error comes:

Possible unhandled promise rejection (id:2). 
TypeError: Invalid attempt to spread non-iterable instance. In order to be iterable, non-array objects must have a [Symbol.iterator()] method.

How can I solve this?