Custom Error Message

#models.py
class Sales(models.Model):
    customer = models.ForeignKey("Customer")
    ctype = models.ForeignKey("Customer_type", verbose_name="Customer Type")

#forms.py
from django.utils.translation import ugettext_lazy as _
class Sales_form(forms.ModelForm):
    class Meta:
        model = Sales
        fields = ('customer','ctype')
        error_messages = {
            'ctype' : {
                'required' : _("This field is required!")
            }
        }

'''
    but the error still returns:
    {"ctype": ["This field is required!"]}

    What I want is like this
    {"Customer Type": ["This field is required!"]}
'''