Monitor.TryEnter Error

[jayy66]

I keep getting the following error in my applictaion:
"Object synchronization method was called from an unsynchronized block of code."

Heres the part of code the error is occurring at:

Public SuccessFax As New SuccessFaxCheck

If Monitor.TryEnter(SuccessFax, 1000) = True Then
                    If aServer.Equals(faxSvr1) Then '1.18
                        'check if there has been at least one successful fax sent
                        If SuccessFax.Svr1Success = True Then
                            If DateDiff(DateInterval.Minute, SuccessFax.sFax1, Now()) > basMod.IntervalBetweenSuccesses Then
                                basMod.SendAlertMail(6, aServer)
                                SuccessFax.Svr1SuccessReceived = False
                                WriteToXML(1, faxSvr1) 'set settings variable to false
                                success = False
                            Else
                                success = True
                            End If
                        Else
                            success = True
                        End If
                    ElseIf aServer.Equals(faxSvr2) Then '10.5
                        'check if there has been at least one successful fax sent
                        If SuccessFax.Svr2Success = True Then
                            If DateDiff(DateInterval.Minute, SuccessFax.sFax2, Now()) > basMod.IntervalBetweenSuccesses Then
                                basMod.SendAlertMail(6, aServer)
                                SuccessFax.Svr2SuccessReceived = False
                                WriteToXML(1, faxSvr2) 'set settings variable to false
                                success = False
                            Else  '<--ERROR OCCURS HERE
                                success = True
                            End If
                        Else
                            success = True
                        End If
                    End If
                    Monitor.Exit(SuccessFax)
                End If

Does anybody see the error of my ways?