This error might occur when you return an object instead of a string in your __unicode__ method. For example:
class Author(models.Model):
. . .
name = models.CharField(...)
class Book(models.Model):
. . .
author = models.ForeignKey(Author, ...)
. . .
def __unicode__(self):
return self.author # <<<<<<<< this causes problems
To avoid this error you can cast the author instance to unicode:
class Book(models.Model):
. . .
def __unicode__(self):
return unicode(self.author) # <<<<<<<< this is OK