Default merge codes not displaying with empty merge field

The issue is in the hyperlink if I have: `...?Name=David&field8=`
(notice how field8 equals nothing)

the default does not display.
`[as what="field8" default="$400"]`

If I remove field8 completely from the hyperlink string, it then works and displays the default.

I want to merge in price but have a default set if I don't input the price I want to quote.

I hope this makes sense. Wondering if a field is in the hyperlink string but equals nothing, if that can then kick in the default in the AS merge code.

David