Atmega2560 USART issue with interrupt...

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#define F_CPU 16000000UL
#include <avr/io.h>
#include <stdint.h>                     // needed for uint8_t
#include <util/delay.h>
#include <avr/interrupt.h>
 
 
#define FOSC 16000000                       // Clock Speed
#define BAUD 9600
#define MYUBRR FOSC/16/BAUD -1
 
 
volatile char ReceivedChar;
 
 
int main( void )
{
    //cli();
    DDRB = 0xFF;    //LED off
    PORTB = 0x00;
    UCSR0B |= (1 << RXEN0) | (1 << TXEN0);      // Enable receiver and transmitter
                        //RX interrupt enabled
    UCSR0C |= (1 << UCSZ01);// | (1 << UCSZ00);    // Set frame: 8data, 1 stp
    
    /*Set baud rate */
    UBRR0H = (( MYUBRR ) >> 8);
    UBRR0L = MYUBRR;
    UCSR0B |= (1 << RXCIE0);
    sei();
 
    
    while(1)
    {
        
        if( ReceivedChar == 'C' || ReceivedChar == 'c'){
            PORTB |= (1<<PORTB5);
            _delay_ms(500);
        }
        
        PORTB = 0x00;
        _delay_ms(500);
        PORTB = 0xFF;
        _delay_ms(500);
        // Main loop
    }
}
 
ISR (USART0_RX_vect , ISR_BLOCK)
{
    
    ReceivedChar = UDR0;
    UDR0 = ReceivedChar;
    //sei();
}