[psycopg] Fetching data from binary cursor

Hi,

I am using Psycopg 2.7.1 from a Django application. To improve the 
performance of some very large queries I want to test server-side binary 
cursors and when I try to fetch results from it, I run into this error:

  rows = cursor.fetchall()
  ValueError: invalid literal for long() with base 10: ''

This is what I do:

  from django.db import connection

  cursor = connection.connection.cursor()
  cursor.execute('''
    DECLARE test_cursor BINARY CURSOR FOR
    SELECT * FROM treenode
  ''')
  cursor.execute("""FETCH 1 FROM test_cursor""")
  rows = cursor.fetchall()

The cursor I get from Django (connection.connection.cursor()) is a 
genuine Psycopg2 cursor:

  > type(cursor)
  psycopg2.extensions.cursor

Is there something I am doing wrong?

Thanks,
Tom