"Frame length 0x${frameLength.toHexString()} > 0x7FFFFFFFFFFFFFFF",
        )
      }
    }

    if (isControlFrame && frameLength > PAYLOAD_BYTE_MAX) {
      throw ProtocolException("Control frame must be less than ${PAYLOAD_BYTE_MAX}B.")
    }

    if (isMasked) {
      // Read the masking key as bytes so that they can be used directly for unmasking.
      source.readFully(maskKey!!)
    }
  }