diff --git a/documentcloud.py b/documentcloud.py
index 75fa2a1..7863d3a 100644
--- a/documentcloud.py
+++ b/documentcloud.py
@@ -311,12 +311,15 @@ class DocumentSet(list):
     from getting into the list and ensuring that only Document
     objects are appended.
     """
-    
-    def append(self, obj, n=1):
-        for i in xrange(n):
-            existing_ids = [i.id for i in list(self.__iter__())]
-            if obj.id not in existing_ids:
-                super(DocumentSet, self).append(copy.copy(obj))
+    def append(self, obj):
+        # Verify that the user is trying to add a Document object
+        if not isinstance(obj, Document):
+            raise TypeError("Only Document objects can be added to the document_list")
+        # Check if the object is already in the list
+        if obj.id in [i.id for i in list(self.__iter__())]:
+            raise DuplicateObjectError("This object already exists in the document_list")
+        # If it's all true, append it.
+        super(DocumentSet, self).append(copy.copy(obj))
 
 
 class Project(BaseAPIObject):
@@ -331,7 +334,7 @@ def __setattr__(self, attr, value):
         if attr == 'document_list':
             if value == None:
                 self.__dict__[u'document_list'] = DocumentSet([])
-            elif type(value) == list:
+            elif isinstance(value, list):
                 self.__dict__[u'document_list'] = DocumentSet(value)
             else:
                 raise TypeError
@@ -497,6 +500,12 @@ class DoesNotExistError(Exception):
     """
     pass
 
+class DuplicateObjectError(Exception):
+    """
+    Raised when the user tries to add a duplicate to a distinct list.
+    """
+    pass
+
 #
 # API connection clients
 #
@@ -716,13 +725,15 @@ def __init__(self, username=None, password=None):
     public = DocumentCloud()
     private = DocumentCloud(DOCUMENTCLOUD_USERNAME, DOCUMENTCLOUD_PASSWORD)
     bad = DocumentCloud("Bad", "Login")
+    # Pull the project
     proj = private.projects.get("703")
     print proj.document_list
     print "docs: %s" % len(proj.document_list)
+    # Zero out the list
     doc = private.documents.get(u'12672-the-klee-report-volume-4')
-    proj.document_list.append(doc)
-    #proj.document_list = None
+    proj.document_list = None
     print proj.document_list
+    # Save the changes and check what sticks
     proj.put()
     proj = private.projects.get("703")
     print "docs: %s" % len(proj.document_list)
diff --git a/test.py b/test.py
index 486d738..23beb23 100644
--- a/test.py
+++ b/test.py
@@ -14,7 +14,7 @@
 import textwrap
 import unittest
 from documentcloud import DocumentCloud
-from documentcloud import CredentialsMissingError
+from documentcloud import CredentialsMissingError, DuplicateObjectError
 from documentcloud import CredentialsFailedError, DoesNotExistError
 from documentcloud import Annotation, Document, Project, Section, Entity
 from private_settings import DOCUMENTCLOUD_USERNAME, DOCUMENTCLOUD_PASSWORD
@@ -270,6 +270,13 @@ def test_put(self):
         obj.put()
         obj = self.private_client.projects.get("703")
         self.assertEqual(len(obj.document_list), len(proj_ids))
+    
+    def test_document_list_type_restrictions(self):
+        """
+        Make sure document_lists will only accept Document objects
+        """
+        obj = self.private_client.projects.get("703")
+        self.assertRaises(TypeError, obj.document_list.append, "The letter C")
 
 
 class ErrorTest(BaseTest):
@@ -291,6 +298,14 @@ def test_does_not_exist(self):
         Make sure DoesNotExistError works.
         """
         self.assertRaises(DoesNotExistError, self.public_client.documents.get, 'TK')
+    
+    def test_duplicate_object(self):
+        """
+        Make sure DuplicateObjectError works.
+        """
+        obj = self.private_client.projects.get("703")
+        doc = self.private_client.documents.get(u'12672-the-klee-report-volume-4')
+        self.assertRaises(DuplicateObjectError, obj.document_list.append, doc)
 
 
 if __name__ == '__main__':