我很难用Toplevel打开一个新窗口

这是我在按下打开新窗口的按钮时遇到的错误。

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/tkinter/__init__.py", line 1699, in __call__
    return self.func(*args)
  File "/Users/Jaguar/PycharmProjects/learnpython/chemcalc.py", line 78, in <lambda>
    Button(master, text='Sub', width=3, command=lambda: self.sub_win()).grid(row=2, column=6)
  File "/Users/Jaguar/PycharmProjects/learnpython/chemcalc.py", line 57, in sub_win
    top = Toplevel(self)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/tkinter/__init__.py", line 2336, in __init__
    BaseWidget.__init__(self, master, 'toplevel', cnf, {}, extra)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/tkinter/__init__.py", line 2286, in __init__
    BaseWidget._setup(self, master, cnf)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/tkinter/__init__.py", line 2256, in _setup
    self.tk = master.tk
AttributeError: 'calc' object has no attribute 'tk'

我的代码:

from tkinter import *


class calc:

    def sub_win(self):
        top = Toplevel(self)


    def __init__(self, master):
        master.title('Title')
        master.geometry()
        self.e = Entry(master, justify = RIGHT)
        self.e.grid(row=0, column=0, columnspan=8, pady=3)
        self.answerlist = []

root = Tk()
obj=calc(root)
root.mainloop()