TypeError: Cannot read properties of undefined (reading 'toString') in ReactJS

currently I am working on a Search Component in ReactJS. In this case I get an "TypeError: Cannot read properties of undefined (reading 'toString')" - Error, while trying to display it on localhost:3000.

If I remove the [newItem], there would not be thrown an error. It would be very cool, if someone could help.

Mainly the problem is the function search(items)..

function App() {
const [error, setError] = useState(null);
const [isLoaded, setIsLoaded] = useState(false);
const [items, setItems] = useState([]);
const [q, setQ] = useState("");
const [searchParam] = useState(["capital", "name"]);

  useEffect(() => {
    fetch("URL")
        .then((res) => res.json())
        .then(
            (result) => {
                setIsLoaded(true);
                setItems(result);

            (error) => {
                setIsLoaded(true);
                setError(error);
            }
        );
}, []);

function search(items) {
    return items.filter((item) => {
        return searchParam.some((newItem) => {
            return (
                item[newItem]
                    .toString()
                    .toLowerCase()
                    .indexOf(q.toLowerCase()) > -1
            );
        });
    });
}

if (error) {
    return <>{error.message}</>;
} else if (!isLoaded) {
    return <>loading...</>;
} else {
    return (
        <div className="wrapper">
            <div className="search-wrapper">
                <label htmlFor="search-form">
                    <input
                        type="search"
                        name="search-form"
                        id="search-form"
                        className="search-input"
                        placeholder="Search for..."
                        value={q}
                        onChange={(e) => setQ(e.target.value)}
                    />
                    <span className="sr-only">Search countries here</span>
                </label>
            </div>
            <ul className="card-grid">
                {search(items).map((item) => (
                    <li>
                        <article className="card" key={item.callingCodes}>
                            <div className="card-image">
                                <img src={item.flag} alt={item.name} />
                            </div>
                            <div className="card-content">
                                <h2 className="card-name">{item.name}</h2>
                                <ol className="card-list">
                                    <li>
                                        population:{" "}
                                        <span>{item.population}</span>
                                    </li>
                                    <li>
                                        Region: <span>{item.region}</span>
                                    </li>
                                    <li>
                                        Capital: <span>{item.capital}</span>
                                    </li>
                                </ol>
                            </div>
                        </article>
                    </li>
                ))}
            </ul>
        </div>
    );
}

}