Problem with SGDMA in ISR

Just in case I am using the SGDMA block incorrectly, here is my code that sets up a dma transfer:

void process_dma_transfer(	uint16 		descriptor_index, 
							alt_u32* 	source_ptr, 
							alt_u32* 	destination_ptr, 
							bool 		fixed_read, 
							bool 		fixed_write,
							bool		interrupt_protect		)
{
	uint8	result;
	alt_avalon_sgdma_construct_mem_to_mem_desc(	&desc, 	// Pointer to descriptor being constructed
    											&desc,	// Pointer to next descriptor
                         						source_ptr,					// Source address
                         						destination_ptr,			// Destination address
                         						USB_MESSAGE_LEN,			// Bytes to send (512)
                        	 					(int) fixed_read,		 	// fixed read?
                        	 					(int) fixed_write	);  	// fixed write?
    // this bit must be set before each transfer
    desc.control = desc.control | OWNED_BY_HW_BIT;
    
	if (interrupt_protect == TRUE)
	{	
		// disable interrupt during dma transfer because alt_avalon_sgdma_do_sync_transfer()
		// isn't thread-safe and it gets called inside the ISR
		disable_fx2_data_ready_interrupt();
	}
    result = (uint8)alt_avalon_sgdma_do_sync_transfer(DMA, &desc);
	if (interrupt_protect == TRUE)
	{	
		// re-enable interrupt after dma transfer
		enable_fx2_data_ready_interrupt();
	}    
}