Is it illegal to have kernel virtual address == 0 ?

Hello, I've seen some modules where it is tested if virtual address is 0, and if yes, it is acted as if it is an error. But can't virtual address be zero too ? For example, in the following module: https://github.com/khilman/omap-test-dmatest/blob/master/main.c it is checked the returned value of dma_alloc_coherent as following: dma_test[i].src_buf = (unsigned long) dma_alloc_coherent(NULL, buf_size (dma_addr_t *)&dma_test[i].src_buf_phys, GFP_KERNEL | GFP_DMA); if (!dma_test[i].src_buf) { printk("dmatest_init(): get_zeroed_page() failed.\n"); r = -ENOMEM; goto cleanup; } As you can see , when dma_alloc_coherent returns 0 , it is acted as if it is an error. Thank you, Ran