diff --git a/sys/dev/isp/isp_sbus.c b/sys/dev/isp/isp_sbus.c
index ad88a954254e..84eb802253a0 100644
--- a/sys/dev/isp/isp_sbus.c
+++ b/sys/dev/isp/isp_sbus.c
@@ -489,7 +489,7 @@ isp_sbus_mbxdma(struct ispsoftc *isp)
 
 	ISP_UNLOCK(isp);
 
-	if (bus_dma_tag_create(NULL, 1, BUS_SPACE_MAXADDR_24BIT-1,
+	if (bus_dma_tag_create(NULL, 1, BUS_SPACE_MAXADDR_24BIT+1,
 	    BUS_SPACE_MAXADDR_32BIT, BUS_SPACE_MAXADDR_32BIT,
 	    NULL, NULL, BUS_SPACE_MAXSIZE_32BIT, ISP_NSEGS,
 	    BUS_SPACE_MAXADDR_24BIT, 0, busdma_lock_mutex, &Giant,
@@ -522,7 +522,7 @@ isp_sbus_mbxdma(struct ispsoftc *isp)
 	len += ISP_QUEUE_SIZE(RESULT_QUEUE_LEN(isp));
 
 	ns = (len / PAGE_SIZE) + 1;
-	if (bus_dma_tag_create(sbs->dmat, QENTRY_LEN, BUS_SPACE_MAXADDR_24BIT-1,
+	if (bus_dma_tag_create(sbs->dmat, QENTRY_LEN, BUS_SPACE_MAXADDR_24BIT+1,
 	    BUS_SPACE_MAXADDR_32BIT, BUS_SPACE_MAXADDR_32BIT, NULL, NULL,
 	    len, ns, BUS_SPACE_MAXADDR_24BIT, 0, busdma_lock_mutex, &Giant,
 	    &isp->isp_cdmat)) {