[Python] Validation error with Django model JSONField

I have added a JSONField to a django model. The data gets saved in dictionary format. But when I try to use that field in admin, it says:

Validation Error: [u'Enter valid JSON']


What does this error mean?

models.py:

udf5 = JSONField(_('UDF5'), max_length=255, blank=True, null=True)

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