Array index crash using signed paks

I’ve got my game making pak files for mobile updates. On launch, we download the pak file and mount the content. When I set the project to sign the pak files - using the Crypto settings in Project Settings - I get a crash on mounting:

06-07 14:36:20.124 13025 13044 D UE4     : [2018.06.07-21.36.20:124][250]LogAndroid: Error: Fatal error: [File:D:\Build\++UE4+Release-4.19+Compile\Sync\Engine\Source\Runtime\PakFile\Private\SignedArchiveReader.cpp] [Line: 46]
06-07 14:36:20.124 13025 13044 D UE4     : LogAndroid: Error: Couldn't find pak signature file '/storage/emulated/0/Android/data/com.HalfCan.BrutesLootz/files/DLC/Content/TestHarness-Android_Multi.sig'

I’m assuming this means I am successfully signing the paks. However, if I manually put the sig file where the error says it expects it, I get an array index crash:

06-07 14:38:28.926 13131 13215 D UE4     : LogAndroid: Error: Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:Runtime/Core/Public\Containers/Array.h] [Line: 610]
06-07 14:38:28.926 13131 13215 D UE4     : LogAndroid: Error: Array index out of bounds: 2975 from an array of size 1910
06-07 14:38:28.926 13131 13215 D UE4     : LogAndroid: Error: [Callstack] 0x00000000BB4A0D10 libUE4.so!StaticFailDebug(wchar_t const*, char const*, int, wchar_t const*, bool)  []
06-07 14:38:28.926 13131 13215 D UE4     : LogAndroid: Error: [Callstack] 0x00000000BB4A0D10 libUE4.so!FDebug::LogAssertFailedMessageImpl(char const*, char const*, int, wchar_t const*, ...)  []
06-07 14:38:28.926 13131 13215 D UE4     : LogAndroid: Error: [Callstack] 0x00000000BB4A0D10 libUE4.so!FChunkCacheWorker::CheckSignature(FChunkRequest const&)  []

Can anyone tell me what I missed?