Objective-C: strlen returning wrong values

Question

So I'm using the random string generation solution from this question.

Then I'm saving the result to the keychain, except that I get EXC_BAD_ACCESS(code=EXC_I386_GPFLT), and the result variable containing OSStatus is 32767.

What am I doing wrong? I have tried running strlen() on both characters and generatedKey - it always returns some ridiculous numbers, as well as running simple [characters length]. I believe there is the problem - it should return 256.

I know this is probably silly, but I'm lost and Google does not help.

int len = 256;

NSString *characters = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
NSMutableString *generatedKey = [NSMutableString stringWithCapacity: len];
for (int i=0; i<len; i++) {
    [generatedKey appendFormat: @"%C", [characters characterAtIndex: arc4random_uniform([characters length]) % [characters length]]];
}

const char *encryptedPass = [generatedKey UTF8String];
OSStatus result = SecKeychainItemCreateFromContent(kSecGenericPasswordItemClass, &list, len, encryptedPass, someKeychain, accessRef, &someKeychainItem);