[erlang-questions] mnesia:dirty_slot/2 returns [] for non-empty table?

[Rick Pettit]

I am maintaining yaws code which presents a user with a list of local
mnesia tables, presenting them with an HTML link for each table.

Upon clicking the link for a given table, the user is either presented
with a dump of the entire table (for "small" tables in which size <= 50),
or a dump of the first "slot" (i.e. slot 0).

This code has worked well for quite some time. Recently, however, someone
brought it to my attention that for a particular table the "slot 0"
display showed [] (even though the table was non-empty).

Here's some poking around I did at the shell:

  (foo_rel@REDACTED)1> mnesia:table_info(test_tab,size).
  4430
  (foo_rel@REDACTED)2> mnesia:dirty_slot(test_tab,0).
  []
  (foo_rel@REDACTED)3> mnesia:dirty_first(test_tab).
  1453
  (foo_rel@REDACTED)4> ets:slot(test_tab,0).
  []
  (foo_rel@REDACTED)5> ets:first(test_tab).
  1453

===

I've just begun digging through the mnesia source code, so it is probably
a bit premature to ask, but does anyone have an idea why
mnesia:dirty_slot/2 might be returning [] in this case (i.e. where the
table is non-empty and where mnesia:dirty_first/1 returns a Key) ?

-Rick