Getting error "entry is not a member" while removing an external host from netgroup

Issue

Trying to remove an external host from a net-group gives an error "This entry is not a member"

# ipa netgroup-remove-member srvnetg02 --hosts=alpha
  Netgroup name: srvnetg02
  Description: srvnetg02
  NIS domain name: ipa01.example.com
  External host: spks, delta, spkd, beta, spkc, spkl, alpha, schez, spki2, spki1
  Member Host: prosrv.example.com, prosrv1b.example.com, prosrv1e.example.com, prosrv3.example.com, prosrv3b.example.com, prosrv5b.example.com, prosrv5v.example.com, prosrvt.example.com
  Failed hosts/hostgroups:
    member host: alpha.example.com: This entry is not a member
---------------------------
Number of members removed 0
---------------------------

Though the host is shown as an External host belonging to this netgroup

# ipa netgroup-show srvnetg02 --raw --all
  dn: ipaUniqueID=5fc19258-b6e6-11e6-a97e-5254002f7b2e,cn=ng,cn=alt,dc=example,dc=com
  cn: srvnetg02
  description: srvnetg02
  nisdomainname: ipa01.example.com
  ipauniqueid: 5fc19258-b6e6-11e6-a97e-5254002f7b2e
  externalhost: spks
  externalhost: delta
  externalhost: spkd
  externalhost: beta
  externalhost: spkc
  externalhost: spkl
  externalhost: alpha
  externalhost: schez
  externalhost: spki2
  externalhost: spki1
  memberhost: fqdn=prosrv.example.com,cn=computers,cn=accounts,dc=example,dc=com
  memberhost: fqdn=prosrv1b.example.com,cn=computers,cn=accounts,dc=example,dc=com
  memberhost: fqdn=prosrv1e.example.com,cn=computers,cn=accounts,dc=example,dc=com
  memberhost: fqdn=prosrv3.example.com,cn=computers,cn=accounts,dc=example,dc=com
  memberhost: fqdn=prosrv3b.example.com,cn=computers,cn=accounts,dc=example,dc=com
  memberhost: fqdn=prosrv5b.example.com,cn=computers,cn=accounts,dc=example,dc=com
  memberhost: fqdn=prosrv5v.example.com,cn=computers,cn=accounts,dc=example,dc=com
  memberhost: fqdn=prosrvt.example.com,cn=computers,cn=accounts,dc=example,dc=com
  objectclass: ipaobject
  objectclass: ipaassociation
  objectclass: ipanisnetgroup