MySQL adapter bug? Date col being set to NULL when default is 0000-00-00

Ok, simple case. MySQL table with date column. The date column default
is 0000-00-00.

When I view these rows in say SQLyog, all rows in this table have
value of 0000-00-00. by default.

Now, onto my Rails app. I don't even care about the date column
described above. I am attempting to update an entirely different
column in the same table.

However, when I use an active record to update the other column, my
update fails because ActiveRecord is submitting "NULL" for the date
column in question.

ActiveRecord::StatementInvalid (Mysql::Error: Column was set to data
type implicit default; NULL supplied for NOT NULL column
'birthday_search' at row 1: UPDATE vb_user SET `birthday_search` =

What is happening here? This seems like an obvious bug, no?