Model user with pk 1L does not exist at Admin

Here is my model:

class UserSIMS(models.Model):

    TATA_USAHA = 0

    DOSEN = 1

    MAHASISWA = 2

    TYPE_CHOICES = ((TATA_USAHA, 'Tata Usaha'), (DOSEN, 'Dosen'), (MAHASISWA, 'Mahasiswa'))

    user = models.OneToOneField(User)

    type = models.IntegerField(choices=TYPE_CHOICES) # 0 = TU 1 = dosen 2 = mahasiswa

class Group(models.Model):

    name = models.CharField(max_length=100)

    lecturer = models.ForeignKey(User, null=True, blank=True, limit_choices_to={'usersims': UserSIMS.DOSEN})

    company = models.ForeignKey(Company, null=True, blank=True)

    tor = models.FileField(upload_to='tor', null=True, blank=True)

    logact = models.FileField(upload_to='logact', null=True, blank=True)

    document = models.FileField(upload_to='document', null=True, blank=True)

    created_at = models.DateTimeField(null=True, blank=True)

    updated_at = models.DateTimeField(null=True, blank=True)

    def save(self, *args, **kwargs):

        if not self.id:

            self.created_at = datetime.datetime.today()

        self.updated_at = datetime.datetime.today()

        super(Group, self).save(*args, **kwargs)

And this is my group admin model

class GroupAdmin(admin.ModelAdmin):

    list_display = ("name", "lecturer", "company")

    def formfield_for_foreignkey(self, db_field, request, **kwargs):

        if db_field.name == "lecturer":

            kwargs["queryset"] = User.objects.filter(usersims__type=UserSIMS.DOSEN)

        return super(GroupAdmin, self).formfield_for_foreignkey(db_field, request, **kwargs)

admin.site.register(Group, GroupAdmin)

When i tried to update the group or add a new group, there are error like this "Model user with pk 1L does not exist". 

What should I do? Thanks in advance. :D

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