Default placeholder for missing fields doesn't work with block in xtract tool

Question

[Crunk]

I am using the xtract tool of the Entrez Direct (EDirect) package of NBCI. I have a list of Accession Numbers:

U47804, U47803, U47802, U47801, U47800, U47799, U47798, X92938

and try to get information about:

1. Name of organism
2. Product of gene
3. Public year

I executed the following command:

for i in {U47804 U47803 U47802 U47801 U47800 U47799 U47798 X92938}; do \
    efetch -db "nucleotide" -id "$i" -format full; done |
    xtract -pattern Seq-entry -element Org-ref_taxname \
              -block Prot-ref_name -def "-" -element Prot-ref_name_E \
              -block Date-std -position first -element Date-std_year

The result I got:

Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  1995

Because the X92938 doesn't have <Prot-ref_name>, so I was expecting to replace the missing fields with "-" by the -def tag of the extract tool. It means my desired result is as follows:

Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1  gp120   1996
Human immunodeficiency virus 1    -     1995

Can someone have experience with the NBCI tool may help me? Thanks a lot.