diff --git a/sipsimple/payloads/imdn.py b/sipsimple/payloads/imdn.py
index dc25c4bb..485e9f94 100644
--- a/sipsimple/payloads/imdn.py
+++ b/sipsimple/payloads/imdn.py
@@ -1,199 +1,199 @@
 
 """Parses and produces IMDN messages according to RFC5438."""
 
 
 __all__ = ['namespace',
            'IMDNDocument',
            'MessageId',
            'DateTime',
            'RecipientUri',
            'OriginalRecipientUri',
            'Subject',
            'Delivered',
            'Failed',
            'Displayed',
            'Processed',
            'Stored',
            'Error',
            'Forbidden',
            'DeliveryStatus',
            'DisplayStatus',
            'ProcessingStatus',
            'DeliveryNotification',
            'DisplayNotification',
            'ProcessingNotification',
            'IMDNMessage']
 
 
 from sipsimple.payloads import XMLDocument, XMLRootElement, XMLAnyURIElement, XMLStringElement, XMLElementChild, XMLElementChoiceChild, XMLEmptyElement, XMLElement
 
 
 namespace = 'urn:ietf:params:xml:ns:imdn'
 
 
 class IMDNDocument(XMLDocument):
     content_type = "message/imdn+xml"
 
 
 IMDNDocument.register_namespace(namespace, prefix=None, schema='imdn.xsd')
 
 
 # Elements
 
 class MessageId(XMLStringElement):
     _xml_tag = 'message-id'
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
 
 class DateTime(XMLStringElement):
     _xml_tag = 'datetime'
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
 
 class RecipientUri(XMLAnyURIElement):
     _xml_tag = 'recipient-uri'
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
 
 class OriginalRecipientUri(XMLAnyURIElement):
     _xml_tag = 'original-recipient-uri'
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
 
 class Subject(XMLStringElement):
     _xml_tag = 'subject'
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
 
 class StatusElement(XMLEmptyElement):
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
     def __init__(self):
         XMLEmptyElement.__init__(self)
 
     def __str__(self):
         return "%s" % (self._xml_tag)
 
 
 class Delivered(StatusElement):
     _xml_tag = 'delivered'
 
 
 class Failed(StatusElement):
     _xml_tag = 'failed'
 
 
 class Displayed(StatusElement):
     _xml_tag = 'displayed'
 
 
 class Processed(StatusElement):
     _xml_tag = 'processed'
 
 
 class Stored(StatusElement):
     _xml_tag = 'stored'
 
 
 class Error(StatusElement):
     _xml_tag = 'error'
 
 
 class Forbidden(StatusElement):
     _xml_tag = 'forbidden'
 
 
 class Status(XMLElement):
     _xml_tag = 'status'
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
     def __init__(self, status):
         XMLElement.__init__(self)
-        if isinstance(status, basestring) and status not in ('delivered', 'failed', 'displayed', 'processed', 'stored', 'forbidden', 'error'):
+        if isinstance(status, str) and status not in ('delivered', 'failed', 'displayed', 'processed', 'stored', 'forbidden', 'error'):
             raise ValueError("illegal value for status")
-        self.status = status if not isinstance(status, basestring) else eval(status.title())()
+        self.status = status if not isinstance(status, str) else eval(status.title())()
 
     def __repr__(self):
         return "%s(%r)" % (self.__class__.__name__, self.status)
 
     def __str__(self):
         return "%s" % (self.status)
 
 
 class DeliveryStatus(Status):
     status = XMLElementChoiceChild('status', types=(Delivered, Failed, Forbidden, Error), required=True, test_equal=True)
 
 
 class DisplayStatus(Status):
     status = XMLElementChoiceChild('status', types=(Displayed, Forbidden, Error), required=True, test_equal=True)
 
 
 class ProcessingStatus(Status):
     status = XMLElementChoiceChild('status', types=(Processed, Stored, Forbidden, Error), required=True, test_equal=True)
 
 
 class NotificationType(XMLElement):
     _xml_namespace = namespace
     _xml_document = IMDNDocument
 
     def __init__(self, status):
         XMLElement.__init__(self)
         self.status = status
 
 
 class DeliveryNotification(NotificationType):
     _xml_tag = 'delivery-notification'
 
     status = XMLElementChild('status', type=DeliveryStatus, required=True, test_equal=True)
 
 
 class DisplayNotification(NotificationType):
     _xml_tag = 'display-notification'
 
     status = XMLElementChild('status', type=DisplayStatus, required=True, test_equal=True)
 
 
 class ProcessingNotification(NotificationType):
     _xml_tag = 'processing-notification'
 
     status = XMLElementChild('status', type=ProcessingStatus, required=True, test_equal=True)
 
 
 # document
 
 class IMDNMessage(XMLRootElement):
     _xml_tag = 'imdn'
     _xml_namespace = namespace
     _xml_document = IMDNDocument
     _xml_children_order = {MessageId.qname: 0,
                            DateTime.qname: 1,
                            RecipientUri.qname: 2,
                            OriginalRecipientUri.qname: 3,
                            Subject.qname: 4,
                            DeliveryNotification.qname: 5,
                            DisplayNotification.qname: 6,
                            ProcessingNotification.qname: 7,
                            None: 8}
 
     message_id = XMLElementChild('message_id', type=MessageId, required=True, test_equal=True)
     datetime = XMLElementChild('datetime', type=DateTime, required=True, test_equal=True)
     recipient_uri = XMLElementChild('recipient_uri', type=RecipientUri, required=False, test_equal=True)
     original_recipient_uri = XMLElementChild('original_recipient_uri', type=OriginalRecipientUri, required=False, test_equal=True)
     subject = XMLElementChild('subject', type=Subject, required=False, test_equal=True)
     notification = XMLElementChoiceChild('notification', types=(DisplayNotification, DeliveryNotification, ProcessingNotification), required=True, test_equal=True)
 
     def __init__(self, message_id=None, datetime=None, recipient_uri=None, original_recipient_uri=None, subject=None, notification=None):
         XMLRootElement.__init__(self)
         self.message_id = message_id
         self.datetime = datetime
         self.recipient_uri = recipient_uri
         self.original_recipient_uri = original_recipient_uri if original_recipient_uri is not None else recipient_uri
         self.subject = subject
         self.notification = notification