Input Form some time return 2 variables list Django

I have simple model with simple form of mail

class ExtraMailData(models.Model):
    client = models.ForeignKey(Client, on_delete=models.DO_NOTHING)
    group = models.CharField(max_length=50)
    mail = models.EmailField(null=True)

    class Meta:
        # make sure we can have uniqe phone number to each client
        unique_together = ('mail', 'group',)

class AddClientMailtData(AjaxFormMixin, forms.ModelForm):
    template_name  = 'client/mail_form.html'
    class Meta:
        model = ExtraMailData
        fields = ('mail',)
        widgets = {
            'mail': forms.EmailInput(attrs={'class':'form-control', 'placeholder': 'Mail'}),

FormsetAddExtraMailData = inlineformset_factory(Client, ExtraMailData,form=AddClientMailtData, extra=1, fields=('mail',), can_delete=True)

Sometimes I get back the value splited to 2 value list , the first contains my data and the second empty:

'extraMail-0-mail': ['', '']

ans sometimes it's return properly.

my current fix is ugly, I check before is_valid() if I have len('extraMail-0-mail') > 2 and if so I replace it's value by 'extraMail-0-mail'[0].

I wonder why is it happening and what can I do to properly fix this?