diff --git a/sys/dev/cardbus/cardbusreg.h b/sys/dev/cardbus/cardbusreg.h
index ec3157e4780e..c356323d2125 100644
--- a/sys/dev/cardbus/cardbusreg.h
+++ b/sys/dev/cardbus/cardbusreg.h
@@ -88,4 +88,4 @@
 #define	CARDBUS_MAPREG_MEM_ADDR(mr)					\
 	((mr) & CARDBUS_MAPREG_MEM_ADDR_MASK)
 #define	CARDBUS_MAPREG_MEM_SIZE(mr)					\
-	(CARDBUS_MAPREG_MEM_ADDR(mr) & -CARDBUS_MAPREG_MEM_ADDR(mr))
+	(CARDBUS_MAPREG_MEM_ADDR(mr) & (~CARDBUS_MAPREG_MEM_ADDR(mr) + 1))