user interface Button not working

[Frankduc]

Hello,

Cant understand why i get this error message. Anyone can give me an explaination?

from tkinter import *

def inverser():
    mot = str(EntreeMot.get())
    inversion = ""

    for carac in mot:
        inversion = carac + inversion
        txtMot.configure(text = str(inversion))

fen1 = Tk()

label1 = Label(fen1, text = "Entrez un mot:")
label1.grid(row = 0, column = 0, sticky = E)

EntreeMot = Entry(fen1)
EntreeMot.grid(row = 0, column = 1)

label2 = Label(fen1, text = "Mot inversé:")
label2.grid(row = 1, column = 0, sticky = E)

txtMot = Label(fen1, text = "")
txtMot.grid(row = 1, column = 1)

bouton = Button(fen1, text = "Inverser", command = inverser)
bouton.grid(row = 2)

fen1.mainloop()

Error:
line 31, in <module>
    bouton = Button(fen1, text = 'Inverser', command = inverser)
NameError: name 'inverser' is not defined

Thank you