diff --git a/drivers/scsi/aha1542.c b/drivers/scsi/aha1542.c
index a9c29757172f7a0f447093dcd4eaa9ebf25e8b04..afb693d7b44f4e52b9d213babd9f971f698b35be 100644
--- a/drivers/scsi/aha1542.c
+++ b/drivers/scsi/aha1542.c
@@ -325,7 +325,7 @@ static irqreturn_t aha1542_interrupt(int irq, void *dev_id)
 			return IRQ_HANDLED;
 		};
 
-		mbo = (scsi2int(mb[mbi].ccbptr) - aha1542->ccb_handle) / sizeof(struct ccb);
+		mbo = (scsi2int(mb[mbi].ccbptr) - (unsigned long)aha1542->ccb_handle) / sizeof(struct ccb);
 		mbistatus = mb[mbi].status;
 		mb[mbi].status = 0;
 		aha1542->aha1542_last_mbi_used = mbi;