LDAP connection problem with self-signed cert

The code I am using:

# Create LDAPObject instance
conn = ldap.initialize(url)
conn.protocol_version=ldap.VERSION3

conn.simple_bind_s(binddn,bindpw)
# This raises:
# ldap.SERVER_DOWN: 
    {'info': 'error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed', 'desc': "Can't contact LDAP server"}

When I use ldap:// instead of ldaps://, it works correctly.

Can anybody help me figure out why this is?

Thanks. :)