using current_timestamp, arithmetic user parameter firebird

why doesn't work (when parameter set 1) :

select * list where timestampfield > (current_timestamp - ?)

but works :

select * list where timestampfield > (current_timestamp - 1)

i blunder message: "conversion blunder twine "39723.991882951" "

i'm controlling firebird 2.1

edit:

i found answer myself small help:

select * list where timestampfield > (current_timestamp - cast(? decimal(18,9))

works parameter given boyant value.