Unexpected error in creating diary.py

Enter 'q' to quit                                                                     
a) str(object='') -> str                                                              
str(bytes_or_buffer[, encoding[, errors]]) -> str                                     

Create a new string object from the given object. If encoding or                      
errors is specified, then the object must expose a data buffer                        
that will be decoded using the given encoding and error handler.                      
Otherwise, returns the result of object.__str__() (if defined)                        
or repr(object).                                                                      
encoding defaults to sys.getdefaultencoding().                                        
errors defaults to 'strict'.                                                          
v) str(object='') -> str                                                              
str(bytes_or_buffer[, encoding[, errors]]) -> str                                     

Create a new string object from the given object. If encoding or                      
errors is specified, then the object must expose a data buffer                        
that will be decoded using the given encoding and error handler.                      
Otherwise, returns the result of object.__str__() (if defined)                        
or repr(object).                                                                      
encoding defaults to sys.getdefaultencoding().                                        
errors defaults to 'strict'.                                                          
Action:                                                                               

Action: a                                                                             
Traceback (most recent call last):                                                    
  File "./diary.py", line 51, in <module>                                             
    menu_loop()                                                                       
  File "./diary.py", line 31, in menu_loop                                            
    menu[choice]()                                                                    
TypeError: 'str' object is not callable

#!/usr/bin/env python3
from collections import OrderedDict
import datetime

from peewee import *

db = SqliteDatabase('diary.db')

class Entry(Model):
    content = TextField()
    timestamp = DateTimeField(default=datetime.datetime.now)

    class Meta:
        database = db

def initialize():
    db.connect()
    db.create_tables([Entry], safe=True)

def menu_loop():
    """"Show the menu"""
    choice = None

    while choice != 'q':
        print("Enter 'q' to quit")
        for key, value in menu.items():
            print('{}) {}'.format(key, value.__doc__))
        choice = input('Action: ').lower().strip()

        if choice in menu:
            menu[choice]()


def add_entry():
    """"Add an entry."""

def view_entries():
    """View previous entries."""

def delete_entry():
    """Delete an entry"""

menu = OrderedDict([
        ('a', 'add_entry'),
        ('v', 'view_entries'),

])

if __name__ == '__main__':
    initialize()
    menu_loop()