NSMutableArray gets dealloc halfway through block - objective-c

Question

I have an NSMutableArray getting populated with values within a enumerateObjects loop. About the 4th or 5th time the function to populate the MutableArray is getting called, I get a SIGSEGV error with the stack
0 libobjc.A.dylib 0x39e7e626 objc_msgSend + 6
1 CoreFoundation 0x2f663dc9 -[__NSArrayM dealloc] + 154
2 libobjc.A.dylib 0x39e83b6b _ZN11objc_object17sidetable_releaseEb + 172
3 MyAppName 0x0004bdff -[BluetoothManager(DataParse) processResponse:] (BluetoothManager+DataParse.m:231)
Setup
An appDelegate which has a strong pointer to BluetoothManager instance.
A function call on BluetoothManager class, with the signature processResponse: on BluetoothManager which is invoked every time didUpdateValueForCharacteristic: is called on the peripheral delegate.
processBleResponse: takes an NSData blob and converts it into an array of NSNumbers
What I'm noticing is that the 4th time or so that processReponse: is called, the mutable Array in which I store all my NSNumbers gets malloc'd within its own enumerateObjects loop.
I've simulated the test with a repeating NSTimer which invokes processResponse: every 30-40s with some dummy data, but the error is reproducible after every few iterations of the loop. The overall memory usage of the app is not spiking over 45MB
// method on BluetoothManager class
- (void)processBleResponse:(NSData *)myData
{
__block float baseValue = 34.500;
__block float incrementValue = 0.0500;
// convert hex data into array of 10 binary "strings"
NSArray *individualBinaryArray = [self extractBinaryValues:myData];
// Extract temperature fields
NSIndexSet *relevantIndices = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(1, individualBinaryArray.count - 1)];
__block NSUInteger indexOfFailure = NSNotFound;
NSMutableArray *decimalValues = [[NSMutableArray alloc] init];
[individualBinaryArray enumerateObjectsAtIndexes:relevantIndices options:NSEnumerationConcurrent usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
NSString *hexString = [self hexString:(NSString *)obj];
if (hexString.length)
{
NSScanner *decimalScanner = [NSScanner scannerWithString:hexString];
unsigned int decimalValue = 0;
if ([decimalScanner scanHexInt:&decimalValue])
{
// insert value
// THIS IS THE LINE WHERE THE CRASH OCCURS
[decimalValues addObject:#(baseValue + incrementValue * decimalValue)];
}
else
{
// throw alert/exception etc.
*stop = YES;
indexOfFailure = idx;
}
}
else
{
*stop = YES;
indexOfFailure = idx;
}
}];
// Insert list of NSNumbers into CoreData
}