Function declared implizit int Fehler bei sprintf unter XC8 mit PIC16F

void main(void) {

    char version;

    char buff[5];

  setupIO();

    setupSPI1();

  setupUART();

    init_MFRC522();

    //delay after Setup

    __delay_ms(20);

  while(1)

  {

      //Get the amount of data waiting in USART queue

      uint8_t n= USARTDataAvailable();

      //If we have some data

      if(n!=0)

      {

          //Read it

          char data=USARTReadData();

          //Use a Switch-Case Structure

          switch (data) {

              case '3': USARTWriteLine("MFRC522 Versionsnr.:");

                        version = readMFRC522(VersionReg);

Zeile 465               sprintf(buff, "%02X", version); //Hex vom  

                                                          RC522:0x91

                        //USARTWriteLine(buff);                       

                      break;

                        ]