USART Interrupt löst nicht aus; warum ?

#include <avr/io.h>

//#include <util/delay.h>

#include <avr/interrupt.h>

//#include <avr/signal.h>

#define BAUD 9600UL      // Baudrate bestimmen

#define UBRR_VAL ((F_CPU+BAUD*8)/(BAUD*16)-1)   // clever runden

volatile unsigned char receive_char;

void init_uart(void);

int main(void)

{

   init_uart();

  sei(); 

  while(1)

  {

  }

   /*while(1)

    {

    UDR = 'z';

    _delay_ms(1000);

    } */

}

/***UART initialisieren***/

void init_uart()

{

    UCSRB |= (1<<RXCIE)|(1<<RXEN)|(1<<TXEN);

    UCSRC |= (3<<UCSZ0); /*UART: 8 Datenbits; keine Parität; 1                    Stopbit => 8N1*/

    UBRRH = UBRR_VAL >> 8;

    UBRRL = UBRR_VAL;

}

ISR(USART_RXC_vect)

{

       receive_char = (UDR);

       UDR = receive_char; // gleich zurücksenden !

}