error in filling data in dtabase through admin page..

from django.db import models
from PIL import Image
from lrntkr.managers import CourseManager

class University(models.Model):
name = models.CharField(max_length=70)
address = models.CharField(max_length=120,blank=True,null=True)
country = models.CharField(max_length=30)
website = models.URLField()
def __unicode__(self):
return (self.name)


class Instructor(models.Model):
name = models.CharField(max_length=70)
specilist = models.CharField(max_length=120)
about = models.CharField(max_length=200,blank=True,null=True)
email = models.EmailField()
universities = models.ManyToManyField(University)
def __unicode__(self):
return self.name

class Course(models.Model):
title = models.CharField(max_length=70)
stream = models.CharField(max_length=70)
universities = models.ManyToManyField(University)
instruct = models.ManyToManyField(Instructor)
description = models.CharField(max_length=70,blank=True, null=True)
start_date = models.CharField(max_length=30)
work_load = models.CharField(max_length=30)
logo = models.ImageField(upload_to='logos',blank=True,null=True)
intro = models.URLField(verify_exists = False, max_length = 225,blank=True,null=True)
initiative = models.URLField(verify_exists = False, max_length = 225,blank=True,null=True)
initiator = models.CharField(max_length=50,blank=True,null=True)
courseinfo = models.BooleanField(db_index=True,default=True)

objects = CourseManager()

def save(self, size=(200, 200)):
if not self.id and not self.logo:
return

super(Course, self).save()

pw = self.logo.width
ph = self.logo.height
nw = size[0]
nh = size[1]

# only do this if the image needs resizing
if (pw, ph) != (nw, nh):
filename = str(self.logo.path)
image = Image.open(filename)
pr = float(pw) / float(ph)
nr = float(nw) / float(nh)

if pr > nr:
# photo aspect is wider than destination ratio
tw = int(round(nh * pr))
image = image.resize((tw, nh), Image.ANTIALIAS)
l = int(round(( tw - nw ) / 2.0))
image = image.crop((l, 0, l + nw, nh))
elif pr < nr:
# photo aspect is taller than destination ratio
th = int(round(nw / pr))
image = image.resize((nw, th), Image.ANTIALIAS)
t = int(round(( th - nh ) / 2.0))
print((0, t, nw, t + nh))
image = image.crop((0, t, nw, t + nh))
else:
# photo aspect matches the destination ratio
image = image.resize(size, Image.ANTIALIAS)

image.save(filename)
def __unicode__(self):
return self.title

GENDER_CHOICES = (
('M', 'Male'),
('F', 'Female'),
)

class Student(models.Model):
name = models.CharField(max_length=70)
birth_date = models.DateField(blank=True, null=True)
email_id = models.EmailField()
contact_no = models.IntegerField(blank=True,null=True)
address = models.CharField(max_length=120,blank=True, null=True)
gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
courses = models.ManyToManyField(Course)

def __unicode__(self):
return self.name

hi, i have got one error during adding data in database though admin page, actually it was working fine, but i made some attributes optional, now it's generating error..
i m adding m models.py ,plz help me..
thanks in advance

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