PIC: Global variable not modified inside interrupt handler

I have an interrupt handler for my PIC 16 that should modify a global variable. The global variable is then read by main() in an infinite loop.

For some reason, it seems that the global variable is being modified inside the interrupt handler, but that the modifications do not persist out of the interrupt handler. I am sure that:

• The interrupt handler is being called properly
• The handler modifies the global variable inside the handler
• The code in main() that reads the global variable is correct

Finally, I have been careful to declare the global variable as volatile as recommended here.

What could explain that the global variable change in the interrupt handler does not persist to main()?

[Note: My C code is compiled by XC8. Below is the full code of my reduced test case.]

#include <pic16f1824.h>
#include <xc.h>

volatile unsigned char buttonPress = 0b0;

void interrupt InterruptServiceRoutine(void) {
    // Update the button press value
    buttonPress = 0b1;
}

void main(void) {
    // Make RA4 an output
    TRISAbits.TRISA4 = 0b0;

    // Turn off status LED
    LATAbits.LATA4 = 0b0;

    // Enable interrupts
    INTCONbits.GIE = 0b1;

    // Enable interrupt-on-change to wake from sleep
    INTCONbits.IOCIE = 0b1;

    // Make the RA2 pin a digital input
    TRISAbits.TRISA2 = 0b1;
    ANSELAbits.ANSA2 = 0b0;

    // Interrupt on RA2 negedges
    IOCANbits.IOCAN2 = 0b1;

    while(1) {
        if(buttonPress == 0b1) {
            // Turn on LED <-- Never turns on
            LATAbits.LATA4 = 0b1;
        }
    }
}