[Tutor] How to display radiobutton window with no buttons selected?

[boB Stepp]

Win7-64bit, Python 3.6.1

When I run the following code, the radiobutton window initially
displays with *all* buttons apparently selected.  However, when the
"Status" button is clicked on, the status is as expected, an empty
string for the checked_radiobutton StringVar().

================================================================================
#!/usr/bin/env python3

import tkinter as tk

root = tk.Tk()

# Report option selected:
def status():
    print('You selected the radiobutton:  %s' % checked_radiobutton.get())
    print()

languages = ('Perl', 'JavaScript', 'PHP', 'Python 2', 'Python 3')

# Create new variable object to keep track of checked radiobutton:
checked_radiobutton = tk.StringVar()

for language in languages:
    # Create new radiobutton:
    tk.Radiobutton(root, text=language, variable=checked_radiobutton,
            value=language).pack(anchor='w')

checked_radiobutton.set('')

tk.Button(root, text='Status', command=status).pack(fill='x')

root.mainloop()
================================================================================

I wish the displayed window to initially display with no button
selected.  What am I missing here?

Thanks!

-- 
boB