CCCrypt difference between iOS5 and iOS6

I have got a decryption/encryption method using CCCrypt() which worked really well on iOS5. Now I am working with the iOS6 SDK and never changed my code, but it seems that something is broken. I can still encrypt a string with a key and decrypt it, but if I use another key to decrypt the same string, the CCCryptStatus coming back from CCCrypt() is still 0(kCCSuccess) - even when the decryption fails, because after that my NSData isn't filled. On iOS5 I got the error message -4303 which I could handle then. Any ideas what can be wrong now?

My Code:

char keyPtr[kCCKeySizeAES256+1]; 
bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

// fetch key data
[key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];

if (encryptOrDecrypt == kCCDecrypt)
{
    data = [GTMBase64 decodeData:data];
}

NSUInteger dataLength = [data length];

size_t bufferSize = dataLength + kCCBlockSizeAES128;

void *buffer = malloc(bufferSize);

size_t numBytesDecrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt(encryptOrDecrypt,
                                      kCCAlgorithmAES128,
                                      kCCOptionPKCS7Padding,
                                      keyPtr,
                                      kCCKeySizeAES256,
                                      NULL ,
                                      [data bytes], dataLength, 
                                      buffer,       bufferSize, 
                                      &numBytesDecrypted);

if (cryptStatus != kCCSuccess){
    // do something, but cryptStatus is always 0!
}

EDIT: Tested it on iPad Simulator 5. When I make a decryption with another key the status I receive is -4303. Only in ios6 the status coming back is 0.