Be more specific in the condition before setting 'fields'

polymorphic/admin.py

@@ -411,7 +411,7 @@ class PolymorphicChildModelAdmin(admin.ModelAdmin):
         kwargs.setdefault('form', self.base_form or self.form)
 
         # prevent infinite recursion in django 1.6+
-        if not self.declared_fieldsets:
+        if 'fields' not in kwargs and not self.declared_fieldsets:
             kwargs['fields'] = None
 
         return super(PolymorphicChildModelAdmin, self).get_form(request, obj, **kwargs)