Select child table id - default to "0" when "Use title" ?

I created a select field type to pick record of child table.

To make it optional, I set the “Use Title” to yes, “Title” and “Title internal value” to blank.

I found that if I pick the blank row on the select list, the saved value is “0”, I think it should be null.

Is it a normal or a bug ? I’m using PostgreSQL 9.3.

Regards,

Almond Wong