Django ValueError at: Cannot assign "": "Pair.male_mutation" must be a "Mutation" instance."

I am new to Django facing this error while saving data into DB:

"ValueError at /breedingRecApp/pairing
Cannot assign "<SpeciesDetail: Black Mask Yellow Chest>": "Pair.male_mutation" must be a "Mutation" instance."

I've 4 foreign keys in Pair model each with specific reason/purposes. when I save data then error come.
in the same error page Form Valid is TRUE and Data What I want is also exactly the same. I don't know why data is not saving in DB.

Note: When I insert the same data from Admin interface it get save without any error it also get updated and delete as well from admin interface.

following are models:

models.py
class Mutation(models.Model):

    species = models.ForeignKey(Species, on_delete=models.CASCADE)
    name = models.CharField(max_length=120, blank=False, null=False)
    description = models.TextField(max_length=300, blank=False, null=False)

    class Meta:
        verbose_name_plural = "Mutations"
        ordering = ('name',)

    def __str__(self):
        return self.name

    def get_absolute_url_delete(self):

        return reverse("breedingRecApp:delete_mutation", kwargs={"mutation_id": self.pk})

class SpeciesDetail(models.Model):

    GENDER_CHOICES = (
        ('M', 'Male'),
        ('F', 'Female'),
    )
    species = models.ForeignKey(Species, on_delete=models.CASCADE)
    mutation = models.ForeignKey(Mutation, on_delete=models.CASCADE)
    fatherName = models.CharField(max_length=120)
    motherName = models.CharField(max_length=120)
    birdBandNo = models.IntegerField()
    cageNo = models.IntegerField()
    picture = models.ImageField(upload_to='pics')
    gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
    health = models.CharField(max_length=120)
    breedingHistory = models.TextField(max_length=300)
    age = models.IntegerField()
    dateAcquired = models.DateTimeField(auto_now_add=False, auto_now=True)

    class Meta:
        verbose_name_plural = "Species Detail"
        ordering = ('species', )

    def __str__(self):
         return str(self.mutation)



class Pair(models.Model):
    species = models.ForeignKey(Species, on_delete=models.CASCADE)
    male_mutation = models.ForeignKey(Mutation, on_delete=models.CASCADE, related_name='male_mutation')
    male = models.ForeignKey(SpeciesDetail, on_delete=models.CASCADE, related_name='male')
    female_mutation = models.ForeignKey(Mutation, on_delete=models.CASCADE, related_name='female_mutation')
    female = models.ForeignKey(SpeciesDetail, on_delete=models.CASCADE, related_name='female')
    boxNo = models.IntegerField()
    cageNo = models.IntegerField()
    dateTimeStamp = models.DateTimeField(auto_now_add=True, auto_now=False)
    description = models.TextField(max_length=300)


    class Meta:
        verbose_name_plural = "Pairs"

    def __str__(self):
        return str(self.male) + ',' + str(self.female)
       
views.py

def pairing(request):
    if request.method == "POST":
        form = Pair_ModelForm(request.POST or None)
        if form.is_valid():
            try:
                form.save()
                return redirect('breedingRecApp:pairs')
            except:
                pass
    else:
        form = Pair_ModelForm()
        context = {'title': 'Pairing',
                   'active': 'active',
                   'model': Pair,
                   'form': form,
                   }
    return render(request, 'breedingRecApp/pairing.html', context)


def load_mutations(request):
    species_id = request.GET.get('species')
    specific_mutations = Mutation.objects.filter(species_id=species_id).order_by('name')
    return render(request, 'breedingRecApp/mutations_dropdown_list_options.html', {'mutations': specific_mutations})

def load_males(request):

    mutation_id = request.GET.get('mutation')
    print(mutation_id)
    males = SpeciesDetail.objects.filter(mutation_id=mutation_id).filter(gender='M').order_by('id')
    print('Male Detail', males)
    return render(request, 'breedingRecApp/males_dropdown_list_options.html', {'males': males})

def load_females(request):

    mutation_id = request.GET.get('mutation')
    print(mutation_id)
    females = SpeciesDetail.objects.filter(mutation_id=mutation_id).filter(gender='F').order_by('id')
    print(females)
    return render(request, 'breedingRecApp/females_dropdown_list_options.html', {'females': females})

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