Weird Undo error when typing on a textbox

I have an app that, whenever my textbox gets focus, and I start typing, I
get the following exception:
"Cannot close undo unit because no opened unit exists" This is from a
holdover app that I had working under the Sept CTP, but none of my textboxes
are currently usable, as they all do this.

Here's the stack trace:
" at MS.Internal.Documents.UndoManager.Close(IParentUndoUnit unit,
UndoCloseAction closeAction)\r\n at
System.Windows.Documents.TextEditorTyping.CloseTypingUndoUnit(TextEditor
This, UndoCloseAction closeAction)\r\n at
System.Windows.Documents.TextEditorTyping.DoTextInput(TextEditor This,
String textData, Boolean isInsertKeyToggled, Boolean
acceptControlCharacters)\r\n at
System.Windows.Documents.TextEditorTyping.TextInputItem.Do()\r\n at
System.Windows.Documents.TextEditorTyping._FlushPendingInputItems(TextEditor
This)\r\n at
System.Windows.Documents.TextEditorTyping.BackgroundInputCallback(Object
This)\r\n at
System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate
callback, Object args, Boolean isSingleParameter)\r\n at
System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source,
Delegate callback, Object args, Boolean isSingleParameter, Delegate
catchHandler)\r\n at
System.Windows.Threading.DispatcherOperation.InvokeImpl()\r\n at
System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object
state)\r\n at System.Threading.ExecutionContext.runTryCode(Object
userData)\r\n at
System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode
code, CleanupCode backoutCode, Object userData)\r\n at
System.Threading.ExecutionContext.RunInternal(ExecutionContext
executionContext, ContextCallback callback, Object state)\r\n at
System.Threading.ExecutionContext.Run(ExecutionContext executionContext,
ContextCallback callback, Object state)\r\n at
System.Windows.Threading.DispatcherOperation.Invoke()\r\n at
System.Windows.Threading.Dispatcher.ProcessQueue()\r\n at
System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg,
IntPtr wParam, IntPtr lParam, Boolean& handled)\r\n at
MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr
lParam, Boolean& handled)\r\n at
MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)\r\n at
System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate
callback, Object args, Boolean isSingleParameter)\r\n at
System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source,
Delegate callback, Object args, Boolean isSingleParameter, Delegate
catchHandler)\r\n at
System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority,
TimeSpan timeout, Delegate method, Object args, Boolean
isSingleParameter)\r\n at
System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority,
Delegate method, Object arg)\r\n at
MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam,
IntPtr lParam)\r\n at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG&
msg)\r\n at
System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)\r\n
at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)\r\n
at System.Windows.Threading.Dispatcher.Run()\r\n at
System.Windows.Application.RunInternal(Window window, String[] args)\r\n
at System.Windows.Application.Run(Window window, String[] args)\r\n at
System.Windows.Application.Run(String[] args)\r\n at
CommandLine.MyApp.Main(String[] args) in C:\\Documents and Settings\\Keith
Patrick\\My Documents\\My Projects\\Bella
Carr\\InfiniteN\\CommandLine\\obj\\Debug\\Source\\MyApp.g.cs:line 51\r\n
at System.AppDomain.nExecuteAssembly(Assembly assembly, String[] args)\r\n
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence
assemblySecurity, String[] args)\r\n at
Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()\r\n at
System.Threading.ThreadHelper.ThreadStart_Context(Object state)\r\n at
System.Threading.ExecutionContext.Run(ExecutionContext executionContext,
ContextCallback callback, Object state)\r\n at
System.Threading.ThreadHelper.ThreadStart()"