Error when send data from ios to nRF51822

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Hi, i tried to send data from ios 7 via tx_characteristics(like in UART example) but received error: didWriteValueForCharacteristic 2A38 error = Error Domain=CBErrorDomain Code=0 "Unknown error." UserInfo=0x165ee7e0 {NSLocalizedDescription=Unknown error.}

What is this error means?

Thanks