diff --git a/CHANGES.rst b/CHANGES.rst
index 9b3c339..453dc48 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -7,6 +7,9 @@ tip (unreleased)
 0.6.0 (2011.02.18)
 ------------------
 
+- Fixed issue #6, bug with InheritanceManager and descriptor fields (e.g.
+  FileField).  Thanks zyegfryed for the fix and sayane for tests.
+
 - updated SplitField to define get_prep_value rather than get_db_prep_value.
   This avoids deprecation warnings on Django trunk/1.3, but makes SplitField
   incompatible with Django versions prior to 1.2.
diff --git a/model_utils/managers.py b/model_utils/managers.py
index 7b764dd..8501e45 100644
--- a/model_utils/managers.py
+++ b/model_utils/managers.py
@@ -2,16 +2,16 @@ from types import ClassType
 
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
-from django.db.models.fields.related import SingleRelatedObjectDescriptor
+from django.db.models.fields.related import OneToOneField
 from django.db.models.manager import Manager
 from django.db.models.query import QuerySet
 
 class InheritanceQuerySet(QuerySet):
     def select_subclasses(self, *subclasses):
         if not subclasses:
-            subclasses = [o for o in dir(self.model)
-                          if isinstance(getattr(self.model, o), SingleRelatedObjectDescriptor)
-                          and issubclass(getattr(self.model,o).related.model, self.model)]
+            subclasses = [rel.var_name for rel in self.model._meta.get_all_related_objects()
+                          if isinstance(rel.field, OneToOneField)
+                          and issubclass(rel.field.model, self.model)]
         new_qs = self.select_related(*subclasses)
         new_qs.subclasses = subclasses
         return new_qs
diff --git a/model_utils/tests/models.py b/model_utils/tests/models.py
index 5e63597..4d54369 100644
--- a/model_utils/tests/models.py
+++ b/model_utils/tests/models.py
@@ -1,3 +1,4 @@
+from datetime import datetime
 from django.db import models
 from django.utils.translation import ugettext_lazy as _
 
@@ -7,21 +8,35 @@ from model_utils.fields import SplitField, MonitorField
 from model_utils import Choices
 
 class InheritParent(InheritanceCastModel):
+    non_related_field_using_descriptor = models.FileField(upload_to='test')
+    normal_field = models.TextField()
     pass
 
 class InheritChild(InheritParent):
+    non_related_field_using_descriptor_2 = models.FileField(upload_to='test')
+    normal_field_2 = models.TextField()
     pass
 
 class InheritChild2(InheritParent):
+    non_related_field_using_descriptor_3 = models.FileField(upload_to='test')
+    normal_field_3 = models.TextField()
     pass
 
 class InheritanceManagerTestParent(models.Model):
+    # test for #6
+    # I'm using FileField, because it will always use descriptor
+    non_related_field_using_descriptor = models.FileField(upload_to='test')
+    normal_field = models.TextField()
     objects = InheritanceManager()
 
 class InheritanceManagerTestChild1(InheritanceManagerTestParent):
+    non_related_field_using_descriptor_2 = models.FileField(upload_to='test')
+    normal_field_2 = models.TextField()
     pass
 
 class InheritanceManagerTestChild2(InheritanceManagerTestParent):
+    non_related_field_using_descriptor_2 = models.FileField(upload_to='test')
+    normal_field_2 = models.TextField()
     pass
 
 class TimeStamp(TimeStampedModel):