from django 1.4 to django 1.5- argument 'verify_exists' what s replacement?-Collection of common programming errors

I’m try to fix an app that worked with django 1.4 within a new installation where I’m using Django 1.5 when I try to syncdb I have:

   TypeError: __init__() got an unexpected keyword argument 'verify_exists'

That’s because my app model.py have inside:

   link = models.URLField(verify_exists=True, max_length=255, null=True, blank=True)

with what I d replace ‘verify_exists’ to make it compatible with Django1.5?