ldap_add: Protocol error (2) additional info: no attributes provided

I'm following this guide to migrate existing users in /etc/passwd and /etc/group on a RHEL6 machine to a new, external OpenLDAP server.

I'm trying to apply this file:

# cat people_group.ldif
dn: ou=People, dc=my_domain, dc=com
  ou: People
  objectclass: organizationalUnit

dn: ou=Group, dc=my_domain, dc=com
  ou: Group
  objectclass: organizationalUnit

I get this error:

# ldapadd -x -W -D "cn=admin,dc=my_domain,dc=com"  -H ldaps://my_hostname.my_domain.com -f people_group.ldif
Enter LDAP Password:
adding new entry "ou=People, dc=my_domain, dc=com ou: People objectclass: organizationalUnit"
ldap_add: Protocol error (2)
    additional info: no attributes provided

If I skip this file and go to add the next file, I get a different error:

# ldapadd -x -W -D "cn=admin,dc=my_domain,dc=com"  -H ldaps://my_hostname.my_domain.com -f group.ldif
Enter LDAP Password:
adding new entry "cn=some_group,ou=Group,dc=my_domain,dc=com"
ldap_add: No such object (32)

I'm guessing it can't find ou=Group, which has to be created by the first command that's giving an error. Is that right?

Here's the first entry in group.ldif:

dn: cn=some_group,ou=Group,dc=my_domain,dc=com
objectClass: posixGroup
objectClass: top
cn: my_domain
userPassword: {crypt}x
gidNumber: 500

Here's the output of ldapsearch on the actual OpenLDAP server:

# ldapsearch -H ldapi:/// -Y EXTERNAL
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
# extended LDIF
#
# LDAPv3
# base <dc=my_domain,dc=com> (default) with scope subtree
# filter: (objectclass=*)
# requesting: ALL
#

# search result
search: 2
result: 32 No such object

# numResponses: 1

Here are additional diagnostics from running the first command:

# ldapadd -x -W -D "cn=admin,dc=my_domain,dc=com"  -H ldaps://my_hostname.my_domain.com -f people_group.ldif -d1
ldap_url_parse_ext(ldaps://my_hostname.my_domain.com)
ldap_create
ldap_url_parse_ext(ldaps://my_hostname.my_domain.com:636/??base)
Enter LDAP Password:
ldap_sasl_bind
ldap_send_initial_request
ldap_new_connection 1 1 0
ldap_int_open_connection
ldap_connect_to_host: TCP my_hostname.my_domain.com:636
ldap_new_socket: 4
ldap_prepare_socket: 4
ldap_connect_to_host: Trying 10.25.223.184:636
ldap_pvt_connect: fd: 4 tm: -1 async: 0
attempting to connect:
connect success
TLS: certdb config: configDir='/etc/pki/tls/certs' tokenDescription='ldap(0)' certPrefix='' keyPrefix='' flags=readOnly
TLS: cannot open certdb '/etc/pki/tls/certs', error -8018:Unknown PKCS #11 error.
TLS: loaded CA certificate file /etc/pki/ca-trust/source/anchors/digicert.intermediate.crt.
TLS: skipping 'renew-dummy-cert' - filename does not have expected format (certificate hash with numeric suffix)
TLS: skipping 'ca-bundle.crt' - filename does not have expected format (certificate hash with numeric suffix)
TLS: skipping 'make-dummy-cert' - filename does not have expected format (certificate hash with numeric suffix)
TLS: skipping 'Makefile' - filename does not have expected format (certificate hash with numeric suffix)
TLS: skipping 'ca-bundle.trust.crt' - filename does not have expected format (certificate hash with numeric suffix)
TLS: certificate [CN=my_hostname.my_domain.com] is not valid - CA cert is not valid
TLS: certificate [CN=my_hostname.my_domain.com] is not valid - error -8172:Peer's certificate issuer has been marked as not trusted by the user..
TLS certificate verification: subject: CN=my_hostname.my_domain.com, issuer: CN=my_hostname.my_domain.com, cipher: AES-256, security level: high, secret key bits: 256, total key bits: 256, cache hits: 0, cache misses: 0, cache not reusable: 0
ldap_open_defconn: successful
ldap_send_server_request
ber_scanf fmt ({it) ber:
ber_scanf fmt ({i) ber:
ber_flush2: 50 bytes to sd 4
ldap_result ld 0x2008420 msgid 1
wait4msg ld 0x2008420 msgid 1 (infinite timeout)
wait4msg continue ld 0x2008420 msgid 1 all 1
** ld 0x2008420 Connections:
* host: my_hostname.my_domain.com  port: 636  (default)
  refcnt: 2  status: Connected
  last used: Thu Nov  5 16:47:00 2015


** ld 0x2008420 Outstanding Requests:
 * msgid 1,  origid 1, status InProgress
   outstanding referrals 0, parent count 0
  ld 0x2008420 request count 1 (abandoned 0)
** ld 0x2008420 Response Queue:
   Empty
  ld 0x2008420 response count 0
ldap_chkResponseList ld 0x2008420 msgid 1 all 1
ldap_chkResponseList returns ld 0x2008420 NULL
ldap_int_select
read1msg: ld 0x2008420 msgid 1 all 1
ber_get_next
ber_get_next: tag 0x30 len 12 contents:
read1msg: ld 0x2008420 msgid 1 message type bind
ber_scanf fmt ({eAA) ber:
read1msg: ld 0x2008420 0 new referrals
read1msg:  mark request completed, ld 0x2008420 msgid 1
request done: ld 0x2008420 msgid 1
res_errno: 0, res_error: <>, res_matched: <>
ldap_free_request (origid 1, msgid 1)
ldap_parse_result
ber_scanf fmt ({iAA) ber:
ber_scanf fmt (}) ber:
ldap_msgfree
adding new entry "ou=People, dc=my_domain, dc=com ou: People objectclass: organizationalUnit"
ldap_add_ext
ldap_send_initial_request
ldap_send_server_request
ber_scanf fmt ({it) ber:
ber_scanf fmt ({) ber:
ber_flush2: 87 bytes to sd 4
ldap_result ld 0x2008420 msgid 2
wait4msg ld 0x2008420 msgid 2 (timeout 100000 usec)
wait4msg continue ld 0x2008420 msgid 2 all 1
** ld 0x2008420 Connections:
* host: my_hostname.my_domain.com  port: 636  (default)
  refcnt: 2  status: Connected
  last used: Thu Nov  5 16:47:00 2015


** ld 0x2008420 Outstanding Requests:
 * msgid 2,  origid 2, status InProgress
   outstanding referrals 0, parent count 0
  ld 0x2008420 request count 1 (abandoned 0)
** ld 0x2008420 Response Queue:
   Empty
  ld 0x2008420 response count 0
ldap_chkResponseList ld 0x2008420 msgid 2 all 1
ldap_chkResponseList returns ld 0x2008420 NULL
ldap_int_select
read1msg: ld 0x2008420 msgid 2 all 1
ber_get_next
ber_get_next: tag 0x30 len 34 contents:
read1msg: ld 0x2008420 msgid 2 message type add
ber_scanf fmt ({eAA) ber:
read1msg: ld 0x2008420 0 new referrals
read1msg:  mark request completed, ld 0x2008420 msgid 2
request done: ld 0x2008420 msgid 2
res_errno: 2, res_error: <no attributes provided>, res_matched: <>
ldap_free_request (origid 2, msgid 2)
ldap_parse_result
ber_scanf fmt ({iAA) ber:
ber_scanf fmt (}) ber:
ldap_msgfree
ldap_err2string
ldap_add: Protocol error (2)
    additional info: no attributes provided

ldap_free_connection 1 1
ldap_send_unbind
ber_flush2: 7 bytes to sd 4
ldap_free_connection: actually freed

What attributes does ldapadd want?

How do I get past the additional info: no attributes provided error?