Defining ISR crashes ATmega328p

I am doing bare metal programming for an ATmega328p on an Arduino UNO R3. I am trying to blink an LED that I connected to the board by using interrupts with output compare through Timer/Counter 1. I wrote the following program.

#include <avr/io.h>
#include <avr/interrupt.h>

ISR (TIMER1_COMPA_vect)
{
    // toggle Port D, Pin 5
    PORTD ^= (1 << PORTD5);

}

int main(void) {

    // write 0 to PRTIM1 bit to enable Timer/Counter 1
    PRR &= ~(1 << PRTIM1);

    // output compare value
    OCR1A = 15625 - 1;

    // disconnect OC1A/OC1B pins by writing 0 to all these pins
    TCCR1A = 0;

    // operate in clear timer on compare (CTC) mode
    // WGM10 & WGM11 are 0 by default
    // select clock source using CS12, CS11, CS10 bits of TCCR1B
    // sets clock to clk_I/O / 1024 preclaser
    // 15625 Hz, so need that many ticks - 1 for 1s
    TCCR1B |= (1 << WGM12);
    TCCR1B |= (1 << CS12);
    TCCR1B &= ~(1 << CS11);
    TCCR1B |= (1 << CS10);

    // enable output compare interrupts for output compare unit A
    TIMSK1 |= (1 << OCIE1A);

    // global interrupt enable
    sei();

    // enable PORTD5 as output and write logic low to start
    DDRD |= (1 << DDD5);
    PORTD &= ~(1 << PORTD5);


    // for debugging purposes; enable onboard LED (pin 13 AKA Port B, Pin 5)
    DDRB |= (1 << DDB5);
    PORTB |= (1 << PORTB5);



    // run
    while (1) {

        // NOP
        ;;
    }

    // keeps compiler happy
    return 0;
}

I enable the onboard LED on the Uno R3 to ensure my program is properly flashed, and that's where things get weird. If I don't have the ISR at the top (either I delete it or comment it out) and flash my code, it works fine; the onboard LED (not the one I connected via Port D, Pin 5) turns on and stays on. Obviously, the interrupt portion of the code doesn't work (i.e. my connected LED won't flash). If I uncomment the ISR to flash the pin when Timer/Counter 1 interrupts the CPU and flash the program, the onboard LED -- which should just stay on -- starts flashing rapidly. The last time the onboard LED flashed like this (on a different project), my program was broken and hadn't flashed properly. I'm assuming it's the same issue now; something in the code is not working right and won't flash. The LED I connected never blinks. I've done some surfing but haven't seen people with this problem yet. Anyone know why my ISR is breaking the program?

It appears to have been my Makefile. I was compiling & flashing with avr-gcc and avrdude. I opened the file in Arduino IDE and wrote the raw C code and flashed via the IDE's built-in upload feature. Everything began to work as expected after that.