Crash when undoing changes

[finestructure]

2015-05-26 12:45:03.658 Xcode[57664:1378241] [MT] DVTAssertions: UNCAUGHT EXCEPTION (NSRangeException): -[__NSCFString substringWithRange:]: Range {14, 6} out of bounds; string length 15
UserInfo: (null)
Hints: 
  0: DVTUndoManager for delegate:<IDEPlaygroundDocument: 0x1199527b0>

CreationBacktrace:

  0  0x000000010077209a -[DVTUndoManager init] (in DVTKit)
  1  0x00000001012417a7 -[IDEEditorDocument undoManager] (in IDEKit)
  2  0x000000010132dd2b -[IDEDocumentController editorDocumentsToManuallySave] (in IDEKit)
  3  0x000000010132dba0 -[IDEDocumentController saveAllEditorDocumentsAsyncronouslyWithCompletionBlock:] (in IDEKit)
  4  0x000000010132db34 -[IDEWorkspaceDocument performBlockAfterPreBuildSave:] (in IDEKit)
  5  0x0000000115a56841 __40-[IDEPlaygroundEditor executePlayground]_block_invoke (in IDELanguageSupportUI)
  6  0x0000000102fd428c -[DVTDeveloperModeAlertHelper askToEnableDeveloperModeIfNecessaryWithCompletionHandler:] (in DVTDeveloperModeHelper)
  7  0x0000000115a5678e -[IDEPlaygroundEditor executePlayground] (in IDELanguageSupportUI)
  8  0x00007fff9698041b __NSFireDelayedPerform (in Foundation)
  9  0x00007fff9a1162e4 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ (in CoreFoundation)
 10  0x00007fff9a115f73 __CFRunLoopDoTimer (in CoreFoundation)
 11  0x00007fff9a18953d __CFRunLoopDoTimers (in CoreFoundation)
 12  0x00007fff9a0d1608 __CFRunLoopRun (in CoreFoundation)
 13  0x00007fff9a0d0bd8 CFRunLoopRunSpecific (in CoreFoundation)
 14  0x00007fff9565856f RunCurrentEventLoopInMode (in HIToolbox)
 15  0x00007fff956582ea ReceiveNextEventCommon (in HIToolbox)
 16  0x00007fff9565812b _BlockUntilNextEventMatchingListInModeWithFilter (in HIToolbox)
 17  0x00007fff910149bb _DPSNextEvent (in AppKit)
 18  0x00007fff91013f68 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] (in AppKit)
 19  0x0000000100761aaa -[DVTApplication nextEventMatchingMask:untilDate:inMode:dequeue:] (in DVTKit)
 20  0x00007fff91009bf3 -[NSApplication run] (in AppKit)
 21  0x00007fff90f86354 NSApplicationMain (in AppKit)
 22  0x00007fff8e8a65c9 start (in libdyld.dylib)
 23  0x0000000000000001