Crash in text to speech using AVSpeechSynthesizer

I have a text to speech feature in my news app, which my users use to read aloud news articles. I have used AVSpeechSynthesizer to this end. I have been seeing quite a lot of crashes in my live app, showing on Crashlytics. I have been able to reproduce this crash neither in a test setup while running the app through XCode, nor while using the live app on my iPhone. Can someone help with this? I am attaching the Crashlytics stack trace here:

#16

Crashed: AXSpeech

EXC_BAD_ACCESS KERN_INVALID_ADDRESS 0x000041a1b76ca3c1

Crashed: AXSpeech

0 libsystem_pthread.dylib 0x1860ff764 pthread_mutex_lock$VARIANT$mp + 354

1 CoreFoundation 0x18639793c CFRunLoopSourceSignal + 68

2 Foundation 0x186de00c4 performQueueDequeue + 464

3 Foundation 0x186ecf218 __NSThreadPerformPerform + 136

4 CoreFoundation 0x18647b77c __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 24

5 CoreFoundation 0x18647b6fc __CFRunLoopDoSource0 + 88

6 CoreFoundation 0x18647afd8 __CFRunLoopDoSources0 + 288

7 CoreFoundation 0x186478b5c __CFRunLoopRun + 1048

8 CoreFoundation 0x186398c58 CFRunLoopRunSpecific + 436

9 Foundation 0x186dcd594 -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 304

10 libAXSpeechManager.dylib 0x195ccf854 -[AXSpeechThread main] + 284

11 Foundation 0x186ecf0f4 __NSThread__start__ + 996

12 libsystem_pthread.dylib 0x1860fc2b4 _pthread_body + 308

13 libsystem_pthread.dylib 0x1860fc180 _pthread_body + 310

14 libsystem_pthread.dylib 0x1860fab74 thread_start + 4