Validating Dates On Forms

Here is the code I have in a module. When I put the correct dates in, the message box appears telling me that the date is incorrect, but I have checked and the closing date appears to be later that the date for completion.

When I accept the error message I get a runtime error 2115 saying that the database engine is being prevented form saving the data to the table.

Can anyone offer some help as to my problem here?

The code:

Private Sub txtbx_Date_Closed_BeforeUpdate(Cancel As Integer)

'Declaring the variables
Dim Completed As String
Dim Closed As String

'Assigning values to the variables
Completed = "txtbx_Date_Completed"
Closed = "txtbx_Date Closed"

'Checking to see if there is no completed date
If IsNull(Completed) Then
MsgBox "The action must be completed prior to being closed"
Me![txtbx_Date_Closed] = Null
Exit Sub
'Validating the closing date
ElseIf Closed < Completed Then
MsgBox "The closing date cannot be before completion date"
Me![txtbx_Date Closed] = Null
Exit Sub
End If

End Sub