diff --git a/main.c b/main.c
index b9eb04ab..2639181a 100644
--- a/main.c
+++ b/main.c
@@ -94,7 +94,7 @@ static void sexp_make_unblocking (sexp ctx, sexp port) {
       sexp_port_flags(port) |= O_NONBLOCK;
 }
 #else
-#define sexp_make_unblocking(ctx, port) 0
+#define sexp_make_unblocking(ctx, port) (void)0
 #endif
 
 static sexp sexp_meta_env (sexp ctx) {