Modules & VBA :: Error (Invalid Use Of Null) In A Function Called From A Form Current Event

I am struggling trying to execute a function inside a Form_current event to display some stats.

The Function is this:

Code:
Function FlightsByAircraft(Aircraft As Long) As Long
Dim rst As DAO.Recordset
Dim dbs As DAO.Database
Dim str As String
str = "SELECT * FROM tblFlights WHERE AircraftID = " & Aircraft

[Code] ....

The code for the Form_Current event is this:

Private Sub Form_Current()
txtStats1 = FlightsByAircraft(Me.AircraftID)

Very simple. Well, the problem is when I move to a new record, a error message comes up: "Run-time error '94' - Invalid use of Null". It is because the AircraftID is not populated at that time. I tried to insert in the function code something like that:

Code:
If IsNull(Aircraft) then
exit function
else
.... (the DAO.Recordset code)

but it doesn't work.