FaultISR - Bulk Device

Hi, this is the function that I call to transmit USB

static unsigned long
Transmit_USB(USBbuffer Data,int size)
{
    unsigned long ulLoop, ulCount,ulLoop_buff;
    unsigned long ulWriteIndex;
    uint32_t buffer_stockage;
    volatile int shiftage;
    tUSBRingBufObject sTxRing;

    //
    // Get the current buffer information to allow us to write directly to
    // the transmit buffer (we already have enough information from the
    // parameters to access the receive buffer directly).
    //
    USBBufferInfoGet(&g_sTxBuffer, &sTxRing);


    //
    // How many characters can we process this time round?
    //
    ulCount = size;

    //
    // Set up to process the characters by directly accessing the USB buffers.
    //
    ulWriteIndex = sTxRing.ulWriteIndex;

    for(ulLoop = 0 ; ulLoop < ulCount ;ulLoop = ulLoop + 4)
    {

    	for(ulLoop_buff = 0 ; ulLoop_buff < 4; ulLoop_buff++){
    		buffer_stockage = Data.data[ulLoop+ulLoop_buff];
    		shiftage = 32 - ( ulLoop_buff + 1 ) * 8;
    		if(shiftage < 0){
    			shiftage = 0;
    		}
    		buffer_stockage = buffer_stockage<<shiftage;
    	}

        //
        // Move to the next character taking care to adjust the pointer for
        // the buffer wrap if necessary.
        //


    	g_pucUSBTxBuffer[ulWriteIndex] = buffer_stockage;
    	ulWriteIndex = ulWriteIndex + 4;
        ulWriteIndex = (ulWriteIndex == BULK_BUFFER_SIZE) ? 0 : ulWriteIndex;

    }
    USBBufferDataWritten(&g_sTxBuffer,size);
    return(ulCount);
}

The function seem to be working because if I check the Txbuffer into the debugger, he got fill with char. But eventually, the MCU fall into FaultISR error and I would like to know why.