Gravando data no firebird em ASP

Boa tarde!

Dim com
Dim a
Dim d
Dim m

function formataData( parametro )
 com = cDate(parametro)
 d = Day(com)
 m = Month(com)
 a = Year(com)

 if d < 10 then d = "0"& d
 if m < 10 then m = "0"& m

 formataData = a &"/"& m &"/"& d
 end function

dim strConexao
dim oRs
dim varpro
dim varper
dim vardatai
dim vardataf
set strConexao = CreateObject("ADODB.Connection")
strConexao.Open("DRIVER=Firebird/InterBase® driver; UID=;PWD=; DBNAME=")

Set oRs = Server.CreateObject("ADODB.Recordset")
varpro = request.form("txtpro")
varper = request.form("txtper")
vardatai = formataData( Request.Form("txtdatai") )
vardataf = formataData( Request.Form("txtdataf") )

Linha 35 do select
Set oRs = strConexao.Execute("insert into pro (produto,PER,DATA_INI,DATA_FIN)values('"&varpro&"','"&varper&"','"&vardatai&"','"&vardataf&"')")

Erro

Microsoft OLE DB Provider for ODBC Drivers erro '80004005'

[ODBC Firebird Driver][Firebird]conversion error from string ""

/promo.asp, linha 35

O é esse erro?

Desde ja agradeço