Strange behaviour while deleting related object

Hello All,

I have a model like this:

class Concept(models.Model):

    tag = models.ForeignKey(Tag, null=True, on_delete=models.SET_NULL)

Now, I've a piece of code in my view:

if concept.tag:

    concept.tag.delete()

concept.save()

I'm getting this error in line concept.save()

ValueError: save() prohibited to prevent data loss due to unsaved related object 'tag'.

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