diff --git a/lib/chibi/signal.c b/lib/chibi/signal.c
index 9f9a190a..379728fa 100644
--- a/lib/chibi/signal.c
+++ b/lib/chibi/signal.c
@@ -18,8 +18,7 @@ static void sexp_call_sigaction (int signum, siginfo_t *info, void *uctx) {
   if (ctx) {
 #if SEXP_USE_GREEN_THREADS
     sexp_global(ctx, SEXP_G_THREADS_SIGNALS) =
-      (sexp) ((sexp_uint_t)sexp_global(ctx, SEXP_G_THREADS_SIGNALS)
-              | (sexp_uint_t)sexp_make_fixnum(1UL<<signum));
+      sexp_make_fixnum((1UL<<signum) | sexp_unbox_fixnum(sexp_global(ctx, SEXP_G_THREADS_SIGNALS)));
 #else
     handler = sexp_vector_ref(sexp_global(ctx, SEXP_G_SIGNAL_HANDLERS),
                               sexp_make_fixnum(signum));