IndexError: list index out of range

I've looked in other questions, and apparently none have the answer, sorry if something is wrong or if you need more information, I'm new.

   def seleciona(self):
    self.posicao = self.mostrar.curselection()
    if self.posicao[0] == '':
        messagebox.showinfo('Alerta','Precisa selecionar musica para tocar')
    else:
        nome = todos[int(self.posicao[0])]