# What is ZTUM? **URL:** \ **Category:** Using Swift\ **Tags:** concurrency, crash, asyncawait, ztum\ **Created:** [August 30, 2022, 7:59am UTC](https://forums.swift.org/t/what-is-ztum/59975 "2022-08-30T07:59:48Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![hakonk](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/hakonk/32/17982_2.png) [@hakonk](https://forums.swift.org/u/hakonk)\ **Post date:** [August 30, 2022, 7:59am UTC](https://forums.swift.org/t/what-is-ztum/59975/1 "2022-08-30T07:59:48Z")
Hi! I recently encountered a crash: ```nohighlight OS Version: tvOS 14.5 Report Version: 104 Exception Type: Unknown (SIGSEGV) Exception Codes: SEGV_ACCERR at 0x0000000000000040 Crashed Thread: 9 Application Specific Information: ZTUM Thread 9 Crashed: 0 libswift_Concurrency.dylib 0x10127d5a4 swift::TaskGroup::offer 1 libswift_Concurrency.dylib 0x10127adb0 swift::AsyncTask::completeFuture 2 libswift_Concurrency.dylib 0x10127bec4 swift_continuation_await 3 libswift_Concurrency.dylib 0x101278bfc swift::runJobInEstablishedExecutorContext ``` Had no luck finding explanations for what "ZTUM" means in general (searched the internet, scanned related binaries, searched the Swift repo), much less what it means when coinciding with `TaskGroup`. Thus, - What does `ZTUM` mean? - How can I approach debugging crashes in `Concurrency.TaskGroup ` like the one in the stack trace? Regards, ---
**Author:** ![Jon\_Shier](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jon_shier/32/921_2.png) [@Jon\_Shier](https://forums.swift.org/u/Jon_Shier)\ **Post date:** [August 30, 2022, 3:42pm UTC](https://forums.swift.org/t/what-is-ztum/59975/2 "2022-08-30T15:42:20Z")
Can't say what `ZTUM` means but this crash looks like a known issue on the back deployed concurrency runtime. You may be able to workaround it by changing the structure of your interaction with `TaskGroup` but I haven't seen any particular solution. ---
**Author:** ![danhalliday](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/danhalliday/32/18264_2.png) [@danhalliday](https://forums.swift.org/u/danhalliday)\ **Post date:** [August 31, 2022, 9:27am UTC](https://forums.swift.org/t/what-is-ztum/59975/3 "2022-08-31T09:27:38Z")
@hakonk This is the following bug: [TaskGroup-Related Crash](https://forums.swift.org/t/taskgroup-related-crash/56425). You can remove your use of `TaskGroup`, or live with it for now — it’s fixed as of iOS 15.4. ---
**Author:** ![hakonk](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/hakonk/32/17982_2.png) [@hakonk](https://forums.swift.org/u/hakonk)\ **Post date:** [August 31, 2022, 10:06am UTC](https://forums.swift.org/t/what-is-ztum/59975/4 "2022-08-31T10:06:33Z")
Thanks for replying @Jon_Shier and @danhalliday. Since the crash happened on a back deployed version of `swift_Concurrency.dylib`, I guess the issue stems from Xcode 13.4.1 not bundling a version of the dylib containing the fix that was deployed to iOS/tvOS \>= 15.4. Is this assumption correct, and if so, will the next Xcode toolchain make sure the fix gets included when back deploying?