Typescript说我的对象没有索引签名，尽管我已经定义了索引签名

Question

我已经创建了一个对象并为其定义了一个接口。 尝试使用方括号表示法更新属性时，打字稿说我的对象没有索引签名，即使我定义了索引签名也是如此。

我的对象

const updatedState:internalStateShape = {
            ...this.state,
            customerDetailsForm: {
                ...this.state.customerDetailsForm,
                formFields: {
                    ...this.state.customerDetailsForm.formFields,
                    [fieldId]: {
                        ...this.state.customerDetailsForm.formFields[fieldId]
                    }
                }
            }
        };

我的internalStateShape接口

interface internalStateShape {
    currentCustomerKey: string,
    customerDetailsFormMode: string,
    customerModalOpenState: boolean,
    searchModalOpenState: boolean,
    searchTerm: string,
    customerDetailsForm: {
        formFields: {
            [key:string]: formFieldShape
        },
        formValid: boolean,
    },
    searchFieldForm: {
        formFields: {
            [key:string]: formFieldShape
        },
        formValid: boolean,
    }
};

我的formFieldShape接口

interface formFieldShape {
    classes: string
    config: {
        id: string,
        type: string,
        placeholder: string,
        label: string
    },
    value: string,
    validation: {
        required: boolean,
        regex: RegExp,
        message: string,
    },
    valid: boolean,
    touched: boolean,
    showError: boolean
};

在internalStateShape.customerDetailsForm.formFields中，我已指定[key：string]作为索引。

Typescript抱怨 Element implicitly has an 'any' type because type '{ "firstName": { classes: string; config: { id: string; type: string; placeholder: string; label: string; }; value: string; validation: { required: boolean; regex: RegExp; message: string; }; valid: boolean; touched: boolean; showError: boolean; }; "lastName": { ...; }; "dateOfBirth": { ...; }; }' has no index signature.ts(7017)

到达时

...this.state.customerDetailsForm.formFields[fieldId]

在我的updatedState对象中

为什么会这样？