bsdtar doesn't work with ports backup of libsigc++20

I was doing a complete rebuild of my system and ran into a problem with
bsdtar:

portupgrade -f libsigc++-2.0.3

--->  Backing up the old version
tar: share/doc/libsigc-2.0/docs/reference/html/classsigc_1_1signal_3_01T__return_00_01T__arg1_00_01T__arg2_00_01T__arg3_00_01T__arg4_00_01T__arg5_00_01T__arg6_01_4-members.html: (null)
** Backup failed.

switching over to gtar and it works.

Cheers,
Sean