Crash in AXSpeechImplementation

I have a very weird crash happening for an app that's last updated several months ago. It works in iOS 9 for me, but for some customers it crashes like this:

Hardware Model:      iPhone7,2
Code Type:       ARM-64
Parent Process:  ??? [1]

Date/Time:       2015-10-07T12:55:29Z
OS Version:      iPhone OS 9.0.2 (13A452)
Report Version:  104

Exception Type:  SIGABRT
Exception Codes: #0 at 0x19ad931e0
Crashed Thread:  0

Application Specific Information:
*** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndex:]: index 9223372036854775807 beyond bounds [0 .. 35]'

Last Exception Backtrace:
0   CoreFoundation                       0x0000000185854f5c __exceptionPreprocess + 124
1   libobjc.A.dylib                      0x000000019a44bf80 objc_exception_throw + 52
2   CoreFoundation                       0x000000018573af68 -[__NSArrayM objectAtIndex:] + 236
3   AXSpeechImplementation               0x000000018372b2d0 0x183728000 + 13008
4   AXSpeechImplementation               0x000000018372ae88 0x183728000 + 11912

The line where it crashes is this:

AVSpeechSynthesisVoice *voice = [AVSpeechSynthesisVoice voiceWithLanguage:localeIdentifier]

where localeIdentifier is whatever is returned by [NSLocale currentLocale].localeIdentifier or forced into "en-US" if that current one is not English.

Anyone from Apple can help with this..?