Runtime Error 2001- You Cancelled The Previous Operation

The code below is from a form named frmWorkorderComplete I use to append the tblWorkordersComplete table with data from the fields in the table tblWorkOrders. I'm trying to prevent duplicate entries in the tblWorkOrdersComplete table. To do this, I create a unique identifier for each record to be appended by combining the "ordered", "company", and "salescategory" into a string which is inserted into the "wonmbr" primary key field of the tblWorkorderComplete.

The following code is used in the subroutine to check if a particular wonmbr has already been appended. However, I keep receiveing the error: "Runtime Error 2001- You cancelled the previous operation" EVERYTHING else in this sub routine works fine when I remove the offending code:

If DCount("[wonmbr]", "tblWorkorderComplete", "[wonmbr] = str_wonmbr") > 0 Then
MsgBox "Workorder Already Appended!"
Exit Sub
Else


///////////////////////////
Private Sub Archive_Click()

' declare variables
Dim Variable1 As String
Dim Variable2 As String
Dim Variable3 As Date
Dim Variable4 As Date
Dim Variable5 As Date
Dim Variable6 As Date
Dim Variable7 As Date
Dim str_wonmbr As String
Dim strI As String
Dim strS As String
Dim strSQL As String

Variable1 = [company]
Variable2 = [salecategory]
Variable3 = [ordered]
Variable4 = [filled]
Variable5 = [billed]
Variable6 = [shipped]
Variable7 = [received]

' Create the unique identifier
str_wonmbr = Format([ordered], "yyyy-mm-dd") & "-" & [company] & "-" & [salescategory]

' HERE IS THE OFFENDING CODE
If DCount("[wonmbr]", "tblWorkorderComplete", "[wonmbr] = str_wonmbr") > 0 Then
MsgBox "Workorder Already Archived!"
Exit Sub
Else

' build SQL string
strI = "INSERT INTO tblWorkordersComplete (wonmbr, company, salescategory, ordered ) "
strS = "SELECT '" & str_wonmbr & "', '" & Variable1 & "', #" & Variable3 & "#, #" & Variable4 & "#, #" & Variable5 & "#, #" & Variable6 & "#, #" & Variable7 & "#;"
strSQL = strI & strS

' run SQL code and append data
DoCmd.RunSQL strSQL

End If
End Sub