dump/dd/tar + gpg > tape = file write error

[User1001]

When attempting to back up a filesystem with GPG to encrypt the stream,
the result (so far) has always been:

gpg: /dev/sa0: write error: Invalid argument
gpg: iobuf_flush failed on close: file write error

no matter what method (dump, dd, tar, etc.) was used to create the "dump
stream". It appears to occur only at the end of the "dump stream".
Different options for dd (conv=notrunc conv=osync) don't make any
difference. Using the "--batch" option for GPG doesn't make any difference.

Is there a way to correct this (without creating any new files)?