diff --git a/signal.c b/signal.c
index f5900e66f8..d71f757ca5 100644
--- a/signal.c
+++ b/signal.c
@@ -935,6 +935,9 @@ sigsegv(int sig SIGINFO_ARG)
 #endif
 
 #ifdef SIGILL
+
+NORETURN(static ruby_sigaction_t sigill);
+
 static RETSIGTYPE
 sigill(int sig SIGINFO_ARG)
 {