I have created a CupertinoFilledButton using the flet library, but the text inside the button is not visible. Is there a solution available?

Container(
    content=CupertinoFilledButton(
        content=ft.Text("OK"),
        opacity_on_click=0.6,
        on_click=lambda e: print("clicked!"),
        alignment=ft.alignment.center,
        height=30,
    ),

Font size was changed to extremely small.

                Container(
                    content=CupertinoFilledButton(
                        content=ft.Text("OK", size=5),
                        opacity_on_click=0.6,
                        on_click=lambda e: print("clicked!"),
                        alignment=ft.alignment.center,
                        height=30,
                    ),

Result: What looked like some of the letters were visible.

(English may be unnatural due to the use of translation tools, sorry)