diff --git a/src/libsystemd/sd-event/sd-event.c b/src/libsystemd/sd-event/sd-event.c
index 52660833b5..703bbee636 100644
--- a/src/libsystemd/sd-event/sd-event.c
+++ b/src/libsystemd/sd-event/sd-event.c
@@ -3429,7 +3429,8 @@ static int process_signal(sd_event *e, struct signal_data *d, uint32_t events, i
                 if (_unlikely_(n != sizeof(si)))
                         return -EIO;
 
-                assert(SIGNAL_VALID(si.ssi_signo));
+                if (_unlikely_(!SIGNAL_VALID(si.ssi_signo)))
+                        return -EINVAL;
 
                 if (e->signal_sources)
                         s = e->signal_sources[si.ssi_signo];