CCS/TM4C123GH6PM: API function PWMGenConfigure stuck at FaultISR infinite loop

Tool/software: Code Composer Studio

I am trying to use PWM on my TI board. I found this problem when I used single step into in the debug mode. My codes are shown below:

#include "inc/tm4c123gh6pm.h"
#include <stdbool.h>
#include <stdint.h>
#include "inc/hw_memmap.h"
#include "driverlib/pwm.h"
#include "driverlib/gpio.h"
#include "inc/hw_GPIO.h"
#include "driverlib/pin_map.h"
#include "driverlib/sysctl.h"


void PWMInitTom(void) {
    SysCtlPeripheralEnable(SYSCTL_PERIPH_PWM0); // Enable the PWM0 peripheral
    //while(!SysCtlPeripheralReady(SYSCTL_PERIPH_PWM0)) {/* Wait for the PWM0 module to be ready.*/}
    SysCtlPWMClockSet(SYSCTL_PWMDIV_1); //Set the PWM clock to the system clock
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOB); // Enable GPIOB
    GPIOPinConfigure(GPIO_PB6_M0PWM0); // configure the GPIO pin to select PWM0 functions
    GPIOPinTypePWM(GPIO_PORTB_BASE, GPIO_PIN_6); // Configure the PWM function for this pin.

    //Configure the PWM generator for count down mode with immediate updates to the parameters.
    PWMGenConfigure(PWM0_BASE, PWM_GEN_0, PWM_GEN_MODE_DOWN | PWM_GEN_MODE_NO_SYNC);

    // Set the period. For a 50 KHz frequency, the period = 1/50,000, or 20
    // microseconds. For a 20 MHz clock, this translates to 400 clock ticks.
    // Use this value to set the period.
    PWMGenPeriodSet(PWM0_BASE, PWM_GEN_0, 400);
    PWMPulseWidthSet(PWM0_BASE, PWM_OUT_0, 100); //Set the pulse width of PWM0 for a 25% duty cycle.
    PWMPulseWidthSet(PWM0_BASE, PWM_OUT_1, 300); //Set the pulse width of PWM1 for a 75% duty cycle.
    PWMGenEnable(PWM0_BASE, PWM_GEN_0); // Start the timers in generator 0
    PWMOutputState(PWM0_BASE, (PWM_OUT_6_BIT | PWM_OUT_7_BIT), true); // Enable the output for PWM6 and PWM7 on Module 0

}

unsigned long freq, width, period, pwmfreq;

int main(void) {
    SysCtlClockSet( SYSCTL_USE_PLL |SYSCTL_SYSDIV_10| SYSCTL_XTAL_16MHZ |SYSCTL_OSC_MAIN); //set clock to 20MHz
    freq = SysCtlClockGet();
    PWMInitTom();
    pwmfreq = SysCtlPWMClockGet();
    width = PWMPulseWidthGet(PWM0_BASE, PWM_OUT_0);
    period = PWMGenPeriodGet(PWM0_BASE, PWM_GEN_0);
    while (1) {

    }
}

So when CCS tried to compile PWMGenConfigure, it would jump into FaultISR and stuck at there forever. 

And when I used the example file (invert.c), I had this same problem. And PWMGenConfigure caused the problem.

I have included pwm.c , sysctl.c , gpio.c to my project. 

Can anyone explain?

Thanks