Google App Engine: TypeError problem with Models

I'm running Google App Engine on the dev server.

Here is my models file:

from google.appengine.ext import db

import pickle
import re

re_dept_code = re.compile(r'[A-Z]{2,}')
re_course_number  = re.compile(r'[0-9]{4}')

class DependencyArcHead(db.Model):
    sink = db.ReferenceProperty()
    tails = db.ListProperty()

class DependencyArcTail(db.Model):
    courses = db.ListProperty()

It gives this error:

Traceback (most recent call last):
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 3192, in _HandleRequest
    self._Dispatch(dispatcher, self.rfile, outfile, env_dict)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 3135, in _Dispatch
    base_env_dict=env_dict)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 516, in Dispatch
    base_env_dict=base_env_dict)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2394, in Dispatch
    self._module_dict)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2304, in ExecuteCGI
    reset_modules = exec_script(handler_path, cgi_path, hook)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2200, in ExecuteOrImportScript
    exec module_code in script_module.__dict__
  File "main.py", line 19, in <module>
    from src.Models import Course, findCourse, validateCourse, dictForJSON, clearAndBu开发者_JS百科ildDependencyGraph
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1279, in Decorate
    return func(self, *args, **kwargs)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1929, in load_module
    return self.FindAndLoadModule(submodule, fullname, search_path)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1279, in Decorate
    return func(self, *args, **kwargs)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1831, in FindAndLoadModule
    description)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1279, in Decorate
    return func(self, *args, **kwargs)
  File "C:\Program Files\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1782, in LoadModuleRestricted
    description)
  File "src\Models.py", line 14, in <module>
    class DependencyArcHead(db.Model):
  File "src\Models.py", line 17, in DependencyArcHead
    tails = db.ListProperty()
TypeError: __init__() takes at least 2 arguments (1 given)

What am I doing wrong?

Possible solution: I was missing the type_name argument in the listProperty() constructor. Oops.