Access 2007 VBA Error 2467 for function - ms-access

Question

Having a very weird problem. I have a function which is called when a form field gets focus. The function is passed four arguments, three of which are values from form fields and the fourth is a global variable which is set when the form loads. The function uses these variables to calculate the value for the field which called the function. This function is called in two different places. Everything was working fine, and now suddenly the function works when called from one place and doesn't work from the other, generating a 2467 run-time error, 'The expression you entered refers to an object that is closed or doesn't exists'. I've checked that the values of the arguments being passed are correct, the function exists OK, so can't see why I'm getting this error. Anyone any ideas?
Private Sub cboFinalStage_GotFocus()
'lookup stage based on TNM values
cboFinalStage = FindStage(cboFinalStageT, cboFinalStageN, cboFinalStageM, gblCancer)
End Sub
Public Function FindStage(cboT As ComboBox, cboN As ComboBox, cboM As ComboBox,
strCancer As String) As String
'use the TNM values entered to find the correct stage for the site and return it
'error handling
If gcfHandleErrors Then On Error GoTo PROC_ERR
'declare variables
Dim strTemp As String
Dim strTable As String
Dim db As DAO.Database
Dim rs As DAO.Recordset
Dim strQuery As String
Dim strSite As String
Dim strSiteFull As String
Dim strT As String
Dim strN As String
Dim strM As String
'initialise variables - if there are no values entered in the 3 comboboxes, exit
'load tumour first in case it isn't already loaded
Forms!frmContainer.subTumour.SourceObject = "fsubTumour"
If Not IsNull(Forms!frmContainer.subTumour.Form!txtICD10) Then
strSite = Left(Forms!frmContainer.subTumour.Form!txtICD10, 3)
strSiteFull = Forms!frmContainer.subTumour.Form!txtICD10
End If
If Not IsNull(cboT.Value) Then
strT = cboT.Value
Debug.Print "T is " & strT
End If
If Not IsNull(cboN.Value) Then
strN = cboN.Value
Debug.Print "N is " & strN
End If
If Not IsNull(cboM.Value) Then
strM = cboM.Value
Debug.Print "M is " & strM
End If
If (IsNull(strT) Or IsNull(strN) Or IsNull(strM)) Then
Debug.Print "null so exiting"
Exit Function
End If
'identify the correct AJCC lookup table by cancer site
Select Case [strCancer]
Case "bla"
strTable = "lkp_AJCC_bladder"
Case "bre"
strTable = "lkp_AJCC_breast"
...
End Select
Debug.Print "AJCC table is " & strTable
'query the AJCC lookup table for the stage
strQuery = "SELECT c_stage FROM " & strTable & " WHERE (((t_value)='" & strT & "')
AND ((n_value)='" & strN & "') AND ((m_value)='" & strM & "'))"
Debug.Print "query is " & strQuery
Set db = CurrentDb
Set rs = db.OpenRecordset(strQuery)
If Not rs.EOF Then
strTemp = rs.Fields(0).Value
Debug.Print "result is " & strTemp
End If
rs.Close
db.Close
'return stage
FindStage = strTemp
'error handling
PROC_EXIT:
Exit Function
PROC_ERR:
If (Err.Number = 2467) Then
MsgBox "Unable to evaluate the stage", vbOKOnly, "Processing error"
Resume PROC_EXIT
Else
MsgBox "Error: (" & Err.Number & ") " & Err.Description, vbCritical
Resume PROC_EXIT
End If
End Function