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NSDictionary allKeys Failure - Unable to understand accident report circumstances

I have the following code:

- (Item *) getRandomItem {
    if (itemIDs == nil) {
        [self parse];
    }
    NSArray * allKeys = [allItems allKeys];
    int seed = arc4random()%[allKeys count];
    return [self getItemByID:[allKeys objectAtIndex:seed]];
}

Sometimes this leads to crashes in a real application, but we cannot reproduce the crash. I tried to analyze the report and understand what could be causing the crash, but I did not succeed. In any case, I try to intervene in the object allItemsto cause a crash, leading to a different error than what is described here.

I would like some help to understand under what circumstances the following failure occurred:

Hardware Model:      iPhone3,1
Code Type:       ARM (Native)
Parent Process:  launchd [1]
OS Version:      iPhone OS 5.0.1 (9A405)
Report Version:  104
Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x00000010
Crashed Thread:  0

Thread 0 name:  Dispatch queue: com.apple.main-thread
Thread 0 Crashed:
0   libobjc.A.dylib                 0x3427eb30 _class_isInitialized
1   libobjc.A.dylib                 0x3427e8d6 _class_initialize
2   libobjc.A.dylib                 0x3427e88e prepareForMethodLookup
3   libobjc.A.dylib                 0x3427e76a lookUpMethod
4   libobjc.A.dylib                 0x3427e008 objc_msgSend_uncached
5   CoreFoundation                  0x33f7c020 CFRetain
6   CoreFoundation                  0x33f85bac +[__NSArrayI __new::]
7   CoreFoundation                  0x33f85ac6 -[__NSPlaceholderArray initWithObjects:count:]
8   CoreFoundation                  0x33f85806 +[NSArray arrayWithObjects:count:]
9   CoreFoundation                  0x33fa0e92 -[NSDictionary allKeys]
10  AClockworkBrain                 0x0008f46e -[ItemManager getRandomItem] (ItemManager.m:360)
......

Thank.

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2 answers
Thread 0 name:  Dispatch queue: com.apple.main-thread
Thread 0 Crashed:
0   libobjc.A.dylib                 0x3427eb30 _class_isInitialized
1   libobjc.A.dylib                 0x3427e8d6 _class_initialize
2   libobjc.A.dylib                 0x3427e88e prepareForMethodLookup
3   libobjc.A.dylib                 0x3427e76a lookUpMethod
4   libobjc.A.dylib                 0x3427e008 objc_msgSend_uncached
5   CoreFoundation                  0x33f7c020 CFRetain
6   CoreFoundation                  0x33f85bac +[__NSArrayI __new::]
7   CoreFoundation                  0x33f85ac6 -[__NSPlaceholderArray initWithObjects:count:]

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NSArray * allKeys = [NSArray arrayWithArray:[allItems allKeys]];
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