diff --git a/includes/Rts.h b/includes/Rts.h
index 45c09f8fb7a373e6345b6750527a97f26a5e219d..3360eda323e52a3b50dbcb748606a7b37b467e4d 100644
--- a/includes/Rts.h
+++ b/includes/Rts.h
@@ -166,7 +166,7 @@ typedef StgInt64 Time;
 
 #if TIME_RESOLUTION == 1000000000
 // I'm being lazy, but it's awkward to define fully general versions of these
-#define TimeToUS(t)      (t / 1000)
+#define TimeToUS(t)      ((t) / 1000)
 #define TimeToNS(t)      (t)
 #define USToTime(t)      ((Time)(t) * 1000)
 #define NSToTime(t)      ((Time)(t))