query model using `filter` or `get` occur ValueError: invalid literal for int() with base 10

[592280502@…]

class User(models.Model):

identifier=models.CharField(max_length=128, unique=True, db_index=True)
..........
..........

class Token(models.Model):

user = models.ForeignKey("User", related_name="Tokens")
token_string = models.CharField(max_length=128, unique=True, db_index=True)

while I already have a tk(<Token object>),I run this:

user = User.objects.filter(id=tk.user_id) if user: user = user[0]

it cause the ValueError: invalid literal for int() with base 10:
File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 113, in nonzero

iter(self).next()

File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 107, in _result_iter

self._fill_cache()

File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 772, in _fill_cache

self._result_cache.append(self._iter.next())

File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 273, in iterator

for row in compiler.results_iter():

File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 680, in results_iter

for rows in self.execute_sql(MULTI):

File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 735, in execute_sql

cursor.execute(sql, params)

File "/usr/local/lib/python2.7/site-packages/django/db/backends/mysql/base.py", line 86, in execute

return self.cursor.execute(query, args)

File "build/bdist.linux-x86_64/egg/MySQLdb/cursors.py", line 174, in execute

self.errorhandler(self, exc, value)

File "build/bdist.linux-x86_64/egg/MySQLdb/connections.py", line 36, in defaulterrorhandler

raise errorclass, errorvalue

ValueError: invalid literal for int() with base 10: '\xd9'
but it disappeared when I restart my project, but a couple of days while running, it occured again
How does it happened and any solutions?

[Russell Keith-Magee]

If you've got a question about how to fix an error, please post to the Django-users mailing list. Trac is a place where we log known bugs so we can make sure they're not forgotten.