ComboBox after update/reload

[pmac]

I've been trying to work through errors in this code in my database(Access 2007) and can't seem to find them. When I select a different record from the combobox I get a messagebox error (which I added to help find the problem). Here's the code:

Function ReloadClientLookup(sClientLookup As String)
   Dim sNewStub As String   ' First chars of ClientLookup.Text
   On Error GoTo ErrMsg
   If Not lockClientLookup Then
      sNewStub = Nz(Left(sClientLookup, conClientLookupMin), "")
      ' If first n chars are the same as previously, do nothing.
      If sNewStub <> sClientLookupStub Then
         If Len(sNewStub) < conClientLookupMin Then
            'Remove the RowSource
            cboClientLookup.RowSource = "SELECT dbo_ClientInfo.clientID, dbo_ClientInfo.[lastName] & "", "" & [firstName] AS fullName FROM dbo_ClientInfo " _
                                        & " WHERE dbo_ClientInfo.lastName = ""false"" "
            sClientLookupStub = ""
         Else
            'New RowSource
            cboClientLookup.RowSource = "SELECT dbo_ClientInfo.clientID, dbo_ClientInfo.[lastName] & "", "" & [firstName] AS fullName FROM dbo_ClientInfo " _
                                        & " WHERE dbo_ClientInfo.lastName Like """ & sNewStub & "*"" " _
                                        & " ORDER BY dbo_ClientInfo.lastName, dbo_ClientInfo.firstName;"
            sClientLookupStub = sNewStub
         End If
      End If
   End If
ErrMsg:
   MsgBox ("Error")
End Function

I've also gotten an error in the afterupdate() method(when I debug sometimes it says "invalid use of null" on clientName = cboClientLookup, and sometimes an error on the line below that:

Private Sub cboClientLookup_AfterUpdate()
   Dim clientName As String
   lockClientLookup = True
   clientName = cboClientLookup
   DoCmd.SearchForRecord acActiveDataObject, , acFirst, "[clientID] = " & Str(Nz(Screen.ActiveControl, 0))
   showAddressDetail (False)
   showEventDetail (False)
   Refresh
   cboClientLookup.SetFocus
   cboClientLookup = clientName
   lockClientLookup = False
End Sub

[June7]

String variables can't be set to Null so if cboClientLookup is passing null to the sub then it will error. Determine why null is passing. What are the circumstances when null is passed? Is user starting to make entry then hits ESC key? Modify code to handle null input.

Don't know what could be wrong with the subsequent line. Is ClientID a number datatype?