ITM debug issue with RTX

Hi:

I'm working on a LPC1768 board with MDK-4.11 and RTX-4.11, I sent some debug messages to
Debug(printf) viewer window simply by ITM_SendChar(), it works well before I use RTX.
but when I use this function in a RTX task, the execution hang in HardFault_Handler of
startup_LPC17xx.s. I can show this with following code snippet from the Keil example
RTX_Blinky.
Somebody pls help out, Thx a lot.

int main (void) {

  SystemInit();                             /* Initialize the MCU clocks     */
  LED_init ();                              /* Initialize the LEDs           */
  GLCD_Init();                              /* Initialize the GLCD           */
  GLCD_Clear(White);                        /* Clear the GLCD                */

/*! debug test begin */
  ITM_SendChar('M');   /* Here,"M" disply in viwer correctlly before RTX init */
/*! debug test end */

  os_sys_init(init);                        /* Initialize RTX and start init */
}

__task void phaseA (void) {
  for (;;) {
    os_evt_wait_and (0x0001, 0xffff);    /* wait for an event flag 0x0001    */
    LED_On (LED_A);
    signal_func (t_phaseB);              /* call common signal function      */
    LED_Off(LED_A);

/*! debug test begin */
  ITM_SendChar('A'); /* Here,"A" doesn't appear in viewer, program hangup in HardFault_Hander*/
/*! debug test end */
  }
}