TypeScript 3:类型中缺少属性

import * as React from 'react';


class ExampleComponent extends React.Component<{}, {
        firstName: string, lastName: string
    }> {
    clearState() {
        this.setState({
            firstName: "",
            lastName: ""
        });
    }

    constructor(props) {
        super(props);
        this.state = {
            firstName: '', lastName: ''
        };

        this.handleChange = this.handleChange.bind(this);
    }

    handleChange(event) {
       //***** This is the line that gives me an error ******
        this.setState({ [event.target.id]: event.target.value });

    }
    public render() {

        return <form>

            <div className="form-group">
                <div className="row">
                    <div className="col">
                        <label className="sr-only">First Name</label>
                        <input type="text" className="form-control name-element" id="firstName"
                            placeholder="First Name" value={this.state.firstName} onChange={this.handleChange} required={true} />
                    </div>

                    <div className="col">
                        <label className="sr-only">Last Name</label>
                        <input type="text" className="form-control name-element" id="lastName"
                            placeholder="Last Name" value={this.state.lastName} onChange={this.handleChange} required={true} />
                    </div>
                </div>
            </div>

        </form>

    }
}

// Wire up the React component to the Redux store
export default ExampleComponent;

Error   TS2345  (TS) Argument of type '{ [x: number]: any; }' is not assignable to parameter of type '{ firstName: string; lastName: string; } | ((prevState: Readonly<{ firstName: string; lastName: string; }>, props: {}) => { firstName: string; lastName: string; } | Pick<{ firstName: string; lastName: string; }, "firstName" | "lastName">) | Pick<...>'.
  Type '{ [x: number]: any; }' is not assignable to type 'Pick<{ firstName: string; lastName: string; }, "firstName" | "lastName">'.
    Property 'firstName' is missing in type '{ [x: number]: any; }'.

如有任何建议,将不胜感激。