Forms :: Runtime Error 3078 - Cannot Find Input Table

I have a form that has a button that calls the following code:

Sub ClosePO_Click()
Dim db As DAO.Database
Dim mvalue As String, strSql as string
Set db = CurrentDb
mvalue = Me.Combo73 'combo box on OpenPO Form
strSql = "UPDATE Print SET OpenPO = NO where [GPO Invoice Number] = '" & mvalue & "'"
Debug.Print strSql
db.Execute dbFailOnError
db.Close
Set db = Nothing
End Sub

When I run (or step through the code, I get the Run-Time Error 3078 ... database cannot find the input table.However, when I copy the debug.print output in the immmediate window and paste into the query builder, the query runs. Here is an example of the debug.print output:

UPDATE [Print] set OpenPO = NO where [GPO Invoice Number] = '40333'
where OpenPO is a yes/no field and [GPO Invoice Number] is a string