React-hook-form with react-select formState not valid but controller don't giving an error

I use react-hook-form for validation. If formState.isValid is false, submit button is disabled. It's working if I don't select an option formState.isValid is return false. But controller don't giving an error. How can I get it to give an error message? Here is the controller code

const { register, errors, formState, control } = useForm({
        mode: "all"
    });

function changeValue(inputValue) {
        onChange({ key: "expiryMonth", value: inputValue.value });
    }
...

{errors.expiryMonth && <span className="input__error-message">required.</span>}
                <Controller
                    name="expiryMonth"
                    defaultValue={null}
                    control={control}
                    value={expiryMonth}
                    render={({ onChange }) => <Select isSearchable={false} placeholder="Month" onChange={(inputValue) =>
                        onChange({ value: changeValue(inputValue) })} options={months} />}
                    rules={{ required: true }}
                    register={register}
                />