diff --git a/coregrind/core.h b/coregrind/core.h
index bbab507..134b1d0 100644
--- a/coregrind/core.h
+++ b/coregrind/core.h
@@ -114,7 +114,7 @@
 
 /* Useful macros */
 /* a - alignment - must be a power of 2 */
-#define ROUNDDN(p, a)	((Addr)(p) & ~((a)-1))
+#define ROUNDDN(p, a)	((Addr)(p) & ~((Addr)(a)-1))
 #define ROUNDUP(p, a)	ROUNDDN((p)+(a)-1, (a))
 #define PGROUNDDN(p)	ROUNDDN(p, VKI_PAGE_SIZE)
 #define PGROUNDUP(p)	ROUNDUP(p, VKI_PAGE_SIZE)