ppp interfaces throw error when adding to team

[TJC]

Attempting to add a ppp interface results in errors:

# ifconfig ppp0 down
# teamdctl team0 port add ppp0
libteamdctl: cli_usock_process_msg: usock: Error message received: "PortAddFail"
libteamdctl: cli_usock_process_msg: usock: Error message content: "Failed to add port."
command call failed (Invalid argument)
#

syslog reports the following:

Nov  4 16:57:36 trusty2 teamd_team0[1084]: Added loop callback: usock_acc_conn, 0xb69000
Nov  4 16:57:36 trusty2 teamd_team0[1084]: usock: calling method "ConfigDump"
Nov  4 16:57:36 trusty2 teamd_team0[1084]: usock: calling method "ConfigDumpActual"
Nov  4 16:57:36 trusty2 teamd_team0[1084]: usock: calling method "StateDump"
Nov  4 16:57:36 trusty2 teamd_team0[1084]: usock: calling method "PortAdd"
Nov  4 16:57:36 trusty2 teamd_team0[1084]: ppp0: Adding port (found ifindex "5").
Nov  4 16:57:36 trusty2 teamd_team0[1084]: ppp0: Failed to add port (Invalid argument).
Nov  4 16:57:36 trusty2 teamd_team0[1084]: Removed loop callback: usock_acc_conn, 0xb69000
Nov  4 16:57:36 trusty2 kernel: [ 1942.040242] team0: Device ppp0 failed to enter team mode
Nov  4 16:57:36 trusty2 kernel: [ 1942.040267] team0: Device ppp0 failed to enter team mode
Nov  4 16:57:36 trusty2 kernel: [ 1942.040292] team0: Device ppp0 failed to enter team mode
Nov  4 16:57:36 trusty2 kernel: [ 1942.040302] team0: Device ppp0 failed to enter team mode