Populate Form Fields On Open

Hi all,

I'm sure this must be a well used function by many access users but I can't find anything about it online at all - please help!

I want to open a subform "AddReservation" and populate three fields with data taken from the relevant record in the main form. So in the btnOpenAddReservation_Click event to open the subform I set global variables, then in the form_open event I have this:

Private Sub Form_Open(Cancel As Integer)

Me.CatID = stLinkCatID
Me.CatName = stLinkCatName
Me.OwnerID = stLinkOwnerID

End Sub

But I get the error 2448 'Can't assign a value to this object'.

However if I put the same three lines into the start of the subform's btnARSave_Click event it works fine:

Private Sub btnARSave_Click()
On Error GoTo Err_btnARSave_Click

Dim stDocName As String
Dim stDocName2 As String
stDocName = "OwnersAndCats"
stDocName2 = "AddReservation"

Me.CatID = stLinkCatID
Me.CatName = stLinkCatName
Me.OwnerID = stLinkOwnerID

DoCmd.DoMenuItem acFormBar, acRecordsMenu, acSaveRecord, , acMenuVer70

YesNo = MsgBox("This reservation has been added successfully, do you want to add another?", vbYesNo + vbQuestion, "Add More Reservations?")
Select Case YesNo
Case vbYes
DoCmd.GoToRecord , , acNext
Case vbNo
DoCmd.Close acForm, stDocName2
DoCmd.Close acForm, stDocName
DoCmd.OpenForm stDocName
DoCmd.GoToRecord , , acGoTo, stRecordNo
End Select

Exit_btnARSave_Click:
Exit Sub

Err_btnARSave_Click:
MsgBox Err.Description
Resume Exit_btnARSave_Click

End Sub

What am I doing wrong? Why can't I populate the fields on open?

Many thanks
PT