When I make two sets of the same radio buttons there will be a dash in one of the buttons created from a function.

import tkinter as tk
from tkinter import ttk


def add_radio_buttons():
    a_b = tk.StringVar()
    a_button = ttk.Radiobutton(window, text="A", value="A", variable=a_b)
    a_button.grid(row=1, column=1)
    a_button.invoke()
    b_button = ttk.Radiobutton(window, text="B", value="B", variable=a_b)
    b_button.grid(row=1, column=2)


window = tk.Tk()

add_radio_buttons()

a_b_too = tk.StringVar()
a_button = ttk.Radiobutton(window, text="A", value="A", variable=a_b_too)
a_button.grid(row=2, column=1)
a_button.invoke()
b_button = ttk.Radiobutton(window, text="B", value="B", variable=a_b_too)
b_button.grid(row=2, column=2)

window.mainloop()

Is this a bug or am I forgetting something?