react/typescript 'string | undefined' is not assignable to type 'string' even I already checked the type

Button

import React from 'react';
import './button.css';

interface ButtonProps {
  primary?: boolean;
  backgroundColor?: string;
  size?: 'small' | 'medium' | 'large';
  label: string;
  onClick?: () => void;
}

export const Button = ({
  primary = false,
  size = 'medium',
  backgroundColor,
  label,
  ...props
}: ButtonProps) => {
  const mode = primary ? 'storybook-button--primary' : 'storybook-button--secondary';
  return (
    <button
      type="button"
      className={['storybook-button', `storybook-button--${size}`, mode].join(' ')}
      style={{ backgroundColor }}
      {...props}
    >
      {label}
    </button>
  );
};

Button.stories

import React from 'react';
import { ComponentStory, ComponentMeta } from '@storybook/react';

import { Button } from '.';

export default {
  title: 'Example/Button',
  component: Button,
  argTypes: {
    backgroundColor: { control: 'color' },
  },
} as ComponentMeta<typeof Button>;

const Template: ComponentStory<typeof Button> = (args) => <Button {...args} />;

export const Primary = Template.bind({});
Primary.args = {
  primary: true,
  label: 'Button',
};

...

Button.test

import { render } from '@testing-library/react';
import { Button } from '.';
import { Primary } from './Button.stories';

test('should render a button', () => {
  render(<Button label="Click Me!" />);
});

test('should render a primary button', () => {
  if (typeof Primary.args === 'undefined') {
    throw new Error('Args is undfeined');
  } else if (typeof Primary.args.label !== 'string') {
    throw new Error('The type of label is not string');
  } else if (Primary.args.label === undefined) {
    return;
  }

  /** 
      Types of property 'label' are incompatible.
      Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.
   */
  render(<Primary {...Primary.args} />);
});

I wrote some test code for the button component that was made by Storybook default. Even though I already checked the type of the label prop if the type is undefined or not before the props are delivered, it throws a syntax error 'string | undefined' is not assignable to type 'string'.

Why did I do wrong?