About MSP432.why does my program always jump into FaultISR()?

the code is one of the example from TI,named  fpu_simple_floating_point_operation.  When I press the button to trigger the interrupt for P1.1  ,the program sometimes jumped into the FaultISR(),but sometimes is normal.who can tell me why??

PS: do you think my English is very bad?

/* DriverLib Includes */
#include "driverlib.h"

/* Standard Includes */
#include <stdint.h>

#include <stdbool.h>
#include <math.h>

/* Statics */
static volatile bool flipFlop;

int main(void)
{
    volatile float fCalculate;
    uint32_t ii;
    flipFlop = false;

    MAP_WDT_A_holdTimer();

    /* Configuring P1.1 as an input and enabling interrupts */
    MAP_GPIO_setAsInputPinWithPullUpResistor(GPIO_PORT_P1, GPIO_PIN1);
    MAP_GPIO_clearInterruptFlag(GPIO_PORT_P1, GPIO_PIN1);
    MAP_GPIO_enableInterrupt(GPIO_PORT_P1, GPIO_PIN1);
    MAP_Interrupt_enableInterrupt(INT_PORT1);
    MAP_Interrupt_enableMaster();

    while(1)
    {
        for(ii=0;ii<20;ii++)
        {
            fCalculate = (sin(50.5) * (12.2f / 50.1f) * 10.22f / 3) * ii;
        }
    }
}

/* GPIO ISR */
void gpio_isr(void)
{
    uint32_t status;

    status = MAP_GPIO_getEnabledInterruptStatus(GPIO_PORT_P1);
    MAP_GPIO_clearInterruptFlag(GPIO_PORT_P1, status);

    /* Toggling the output on the LED */
    if(status & GPIO_PIN1)
    {
        if(flipFlop)
        {
            MAP_FPU_disableModule();
            flipFlop = false;
        }
        else
        {
            MAP_FPU_enableModule();
            flipFlop = true;
        }
    }

}