Double Click Expression Error?

Hello,

I currently have a combo list on a purchase order form that has the following event procedure(s) in it so as to open the inventory window as needed to add/edit inventory.

Not in List:
Private Sub Combo55_NotInList(NewData As String, Response As Integer)
MsgBox "ERROR: ITEM not in list. Double-click this field to add an entry to the list."
Response = acDataErrContinue
End Sub

On Double-Click:
Private Sub Combo55_DblClick(Cancel As Integer)
On Error GoTo Err_Combo55_DblClick
Dim lngCombo55 As Long

If IsNull(Me![Combo55]) Then
Me![Combo55].Text = ""
Else
lngCombo55 = Me![Combo55]
Me![Combo55] = Null
End If
DoCmd.OpenForm "INVENTORYITEMSform", , , , , acDialog, "GotoNew"
Me![Combo55].Requery
If lngCombo55 <> 0 Then Me![Combo55] = lngCombo55

Exit_Combo55_DblClick:
Exit Sub

Err_Combo55_DblClick:
MsgBox Err.Description
Resume Exit_Combo55_DblClick

This works GREAT on other forms I have however, on this particular form it gives me the following error:


ERROR:
You have tried to assign the null value to a variable that is not a variable data type.

I have started from scratch several times....can anyone help? Is there anyway to add a button to the form to open the inventory form and then requery the combo box when closing the inventory form?

Thanks,
Dianne