Django admin: use one-to-one relationship in search_fields

I am trying to to add a search to my model admin list page using the following Model and ModelAdmin classes: models.py ``` from django.contrib.auth.models import User class UserProfile(models.Model): user = models.OneToOneField(User) country = CountryField(blank=True, null=True) ``` admin.py ``` from django.contrib import admin from models import UserProfile class UserProfileAdmin(admin.ModelAdmin): list_display = ('user','country') search_fields = ['user'] ``` But I get the following error while trying to access the UserProfile in admin panel: ``` at /admin/profiles/userprofile/ Related Field has invalid lookup: icontains ``` I have also tried the following: ``` search_fields = ['user_username'] ``` And ``` search_fields = ['user_name'] def user_name(self,obj): return obj.user.username ``` Any solutions?