Marking message as spam emits an Assertion failure

Running SpamSieve 2.8.4, OSX 10.6.6

When I use the hot-keys to mark a message as spam the following Assertion failure appears in the console log:

2/23/11 1:18:55 PM Mail[35783] *** Assertion failure in +[Library flagsChangedForMessages:flags:oldFlagsByMessage:newFlagsByMessage:updateUnreadCount:], /SourceCache/Message/Message-1082/Library.subproj/Library.m:718
This method must be called off the main thread
(
0 Message 0x00007fff80d716e4 -[MFAssertionHandler _handleFailureWithPreamble:description:arguments:] + 137
1 Message 0x00007fff80d71649 -[MFAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 220
2 Message 0x00007fff80c5727f +[Library flagsChangedForMessages:flags:oldFlagsByMessage:newFlagsByMessage:updateUnreadCount:] + 192
3 Message 0x00007fff80c56ba0 -[MessageStore setFlagsFromDictionary:forMessages:] + 1275
4 Message 0x00007fff80c565c5 -[LibraryIMAPStore setFlagsFromDictionary:forMessages:updatingServer:] + 113
5 MailActOn 0x000000010ad92776 -[LibraryIMAPStore_MAO MAOsetFlagsFromDictionary:forMessages:] + 61
6 Message 0x00007fff80c5650d -[MessageStore setJunkMailLevel:forMessages:trainJunkMailDatabase:userRecorded:] + 807
7 Message 0x00007fff80d4aa18 -[Message(ScriptingSupport) setIsJunk:] + 115
8 Foundation 0x00007fff816625f5 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 434
9 Foundation 0x00007fff81756bf1 -[NSObject(NSScriptingInternal) _scriptingSetValue:forKey:] + 116
10 Foundation 0x00007fff8178e0e4 -[NSSetCommand performDefaultImplementation] + 364
11 Foundation 0x00007fff8178a61e -[NSScriptingAppleEventHandler handleCommandEvent:withReplyEvent:] + 222
12 Foundation 0x00007fff81680f62 -[NSAppleEventManager dispatchRawAppleEvent:withRawReply:handlerRefCon:] + 360
13 Foundation 0x00007fff81680d92 _NSAppleEventManagerGenericHandler + 114
14 AE 0x00007fff859ef323 _Z20aeDispatchAppleEventPK6AEDescPS_jPh + 162
15 AE 0x00007fff859ef21c ZL25dispatchEventAndSendReplyPK6AEDescPS + 32
16 AE 0x00007fff859ef123 aeProcessAppleEvent + 210
17 HIToolbox 0x00007fff808d7765 AEProcessAppleEvent + 48
18 AppKit 0x00007fff82c8104b _DPSNextEvent + 1205
19 AppKit 0x00007fff82c807a9 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 155
20 AppKit 0x00007fff82c4648b -[NSApplication run] + 395
21 AppKit 0x00007fff82c3f1a8 NSApplicationMain + 364
22 Mail 0x0000000100002598 0x0 + 4294976920
23 ??? 0x0000000000000002 0x0 + 2
)

I think this is a harmless error that’s caused by a bug in Mac OS X. It seems to be timing-related. Running an AppleScript that sets the junk status of a message can trigger this error, even if SpamSieve is not installed.