Flask SQLAlchemy NOT NULL constraint failed on primary key

I'm trying to create a database in SQLite that has two tables, one for a list of airports, and another for a list of trips between pairs of those airports. I've set it up as a self-referential, many-to-many relationship:

class Trips(db.Model):

    __tablename__ = 'trips'

    id = db.Column(db.Integer, primary_key=True)
    airport_from = db.Column(db.Integer, db.ForeignKey('airport.id'))
    airport_to = db.Column(db.Integer, db.ForeignKey('airport.id'))
    price = db.Column(db.Float)
    date = db.Column(db.Date)

class Airport(db.Model):

    __tablename__ = 'airport'

    id = db.Column(db.Integer, primary_key=True)
    iata = db.Column(db.String(8), index=True, unique=True)
    name = db.Column(db.String(120), index=True, unique=True)
    city = db.Column(db.String(120))
    region = db.Column(db.String(120))
    country = db.Column(db.String(120))

    flying_from = db.relationship('Trips', backref='end', primaryjoin=(id==Trips.airport_to))
    flying_to = db.relationship('Trips', backref='start', primaryjoin=(id==Trips.airport_from))

    def __repr__(self):
        return '<Airport: {0}; IATA: {1}>'.format(self.name, self.iata)

When I open up my Python shell and import these models, I have the SQLAlchemy session add Airport objects and commit just fine, but when I do something like:

>>> t = models.Trips(airport_from=3, airport_to=4, price=230.0)
>>> db.session.add(t)
>>> db.session.commit()

It gives me this traceback:

Traceback (most recent call last): File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context context) File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute cursor.execute(statement, parameters) sqlite3.IntegrityError: NOT NULL constraint failed: trips.id

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/scoping.py", line 150, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 813, in commit
    self.transaction.commit()
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 392, in commit
    self._prepare_impl()
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 372, in _prepare_impl
    self.session.flush()
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 2027, in flush
    self._flush(objects)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 2145, in _flush
    transaction.rollback(_capture_exception=True)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/util/compat.py", line 183, in reraise
    raise value
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/session.py", line 2109, in _flush
    flush_context.execute()
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/unitofwork.py", line 373, in execute
    rec.execute(self)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/unitofwork.py", line 532, in execute
    uow
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/persistence.py", line 174, in save_obj
    mapper, table, insert)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/orm/persistence.py", line 800, in _emit_insert_statements
    execute(statement, params)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
    context)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 1341, in _handle_dbapi_exception
    exc_info
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/util/compat.py", line 189, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=exc_value)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/util/compat.py", line 182, in reraise
    raise value.with_traceback(tb)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
    context)
  File "/Users/heli/nomad/flask/lib/python3.4/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed: trips.id [SQL: 'INSERT INTO trips (airport_from, airport_to, price, date) VALUES (?, ?, ?, ?)'] [parameters: (3, 4, 230.0, None)]

The key part seems to be the bottom line:

sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed: trips.id [SQL: 'INSERT INTO trips (airport_from, airport_to, price, date) VALUES (?, ?, ?, ?)'] [parameters: (3, 4, 230.0, None)]

It looks like because I didn't give a value to the id parameter, that it's refusing to add the object. But I thought that this id would be automatically added and incremented, as happened with the Airport objects' id parameter. What am I missing here?