FX_PTR_ERROR in tx_event_flags_get()

Hi.

My code is:

static FX_MEDIA * p_media;

// Wait until device inserted.
status = tx_event_flags_get (&g_usb_plug_events, EVENT_USB_PLUG_IN, TX_OR_CLEAR, &actual_flags, TX_WAIT_FOREVER);

// Get the pointer to FileX Media Control Block for a USB flash device
p_media = g_fx_media_pd_ptr;

// Retrieve the volume name of the opened media from the Data sector
status = fx_media_volume_get(p_media, volume, FX_DIRECTORY_SECTOR);

if (status == FX_SUCCESS ) {

}

The value of status is FX_PTR_ERROR

I do not understand where the cause of this may be. Can somebody help me?

Thank you