tkinter error. My callback is bypassing the try attempt and giving the except error message. Need calculations to work - tkinter-entry

Question

Hello my callback appears not to be calculating the sales tax and amount due values and is throwing errors. New to tkinter and not sure if I am handling correctly when a button needs to handle multiple commands. Any help would be appreciated!
import tkinter as tk
class TkCalc:
def init(self):
self.main_window = tk.Tk()
self.main_window.title('Tax Calculator')
self.main_window.geometry('250x125')
self.line1_frame = tk.Frame(self.main_window)
self.line2_frame = tk.Frame(self.main_window)
self.line3_frame = tk.Frame(self.main_window)
self.line4_frame = tk.Frame(self.main_window)
self.line5_frame = tk.Frame(self.main_window)
self.purchase_label = tk.Label(self.line1_frame, text='Total Purchase:')
self.purchase_entry = tk.Entry(self.line1_frame, width=10)
self.purchase_label.pack(side='left')
self.purchase_entry.pack(side='left')
self.tax_label = tk.Label(self.line2_frame, text='Tax Rate: ')
self.tax_entry = tk.Entry(self.line2_frame, width=5)
self.tax_label.pack(side='left')
self.tax_entry.pack(side='left')
self.tax_label2 = tk.Label(self.line3_frame, text='Sales Tax: ')
self.tax_value = tk.StringVar()
self.tax_result_label = tk.Label(self.line3_frame, textvariable=self.tax_value)
self.tax_value.set('$ 0.00')
self.tax_label2.pack(side='left')
self.tax_result_label.pack(side='left')
self.amount_due = tk.Label(self.line4_frame, text='Amount Due: ')
self.amount_value = tk.StringVar()
self.amount_due_label = tk.Label(self.line4_frame, textvariable=self.amount_value)
self.amount_value.set('$ 0.00')
self.amount_due.pack(side='left')
self.amount_due_label.pack(side='left')
self.calc_tax_button = tk.Button(self.line5_frame, text='Calculate', command=self.show_tax_message)
self.quit_button = tk.Button(self.line5_frame, text='Quit', command=self.main_window.destroy)
self.calc_tax_button.pack(side='left')
self.quit_button.pack(side='left')
self.line1_frame.pack()
self.line2_frame.pack()
self.line3_frame.pack()
self.line4_frame.pack()
self.line5_frame.pack()
tk.mainloop()
def show_tax_message(self):
try:
purchase_amount = float(self.purchase_entry.get())
tax_amount = float(self.tax_value.get())
tax = purchase_amount * (tax_amount/100)
total = purchase_amount + tax
self.amount_value.set(f'${total:.2f}')
self.tax_value.set(f'${tax:.2f}')
except ValueError:
self.amount_value.set('Error')
self.tax_value.set('Error')
tk_calc = TkCalc()