在tkinter中使用optionmenu小部件时出错

我用python 2.7编写了一段代码,实现了“optionmenu”小部件。我面临一个问题,我在元组中给出了六个值,但当我选择另一个选项而不是“Gmail”时,我在下拉列表中只看到五个值。谁能告诉我我的错误吗?

from Tkinter import *
import ttk

root = Tk()

choices = ("Gmail", "Outlook/Hotmail", "Yahoo", "Comcast", "AT&T", "Verizon")
dropdown_var = StringVar()
dropdown_var.set(choices[0]) # set default value

def data(*args):
    value = dropdown_var.get()
    print(value)

l = ttk.Label(root, text="Select your e-mail provider : ")
l.pack(side="left")

option = ttk.OptionMenu(root, dropdown_var, *choices)
option.pack(side="left", padx=10, pady=10)

b = ttk.Button(root, text="Click", command=data)
b.pack(side="bottom")

root.mainloop()