Este é o meu código:

def ask(what,why):
    root=Tk()
    root.title(why)
    label=Label(root,text=what)
    label.pack()
    entry=Entry(root)
    entry.pack()
    button=Button(root,text='OK',command=root.destroy)
    button.pack()
    root.mainloop()
    return entry.get()

E quando eu chamo o código:

print(ask('Name:','Hello!'))

Eu recebo:

Traceback (most recent call last):
  File "C:\gui.py", line 16, in <module>
    ask('Name:','Hello!')
  File "C:\gui.py", line 15, in ask
    return entry.get()
  File "C:\Python34\lib\tkinter\__init__.py", line 2520, in get
    return self.tk.call(self._w, 'get')
_tkinter.TclError: invalid command name ".48148176"

Estou usando o Python 3.4.3 no Windows 7 de 32 bits.