Redux reducers: items.map is not a function

This is a question with a learning purpose, I am stuck in a specific approach. I am playing a little bit with Redux to learn it and I have a question about a part of my code. Let’s say that I have the component that I am attaching as a screenshot which is pretty simple.

This item property is an object of 4 key value pairs (uuid, quantity, price, name ). I am trying to update the quantity in 2 ways. One is successful one is not.

In the reducer this works:

   if (action.type === QUANTITY_UPDATED) {
    return state.map((item) => {
      console.log(item);
      if (item.uuid === action.payload.uuid) {
        return { ...item, quantity: action.payload.quantity };
      }
    
      return item;
    });
   };

But this does not and return the error I mentioned that .map is not a function. It is probably a stupid mistake I am making but can’t figure it out. Any input?

  if (action.type === QUANTITY_UPDATED) {
    const item = state.find((item) => item.uuid === action.payload.uuid);
    return { ...item, quantity: action.payload.quantity };
  }

Moreover, be sure that in Redux reducers, I can’t mutate the properties. Could it be the reason?