diff --git a/coregrind/m_signals.c b/coregrind/m_signals.c
index 317ee034f..2a7f7e1c4 100644
--- a/coregrind/m_signals.c
+++ b/coregrind/m_signals.c
@@ -1661,6 +1661,8 @@ static int sanitize_si_code(int si_code)
    return (Short)si_code;
 #elif defined(VGO_aix5)
    return si_code;
+#else
+#  error Unknown OS
 #endif
 }