diff --git a/degesch.c b/degesch.c
index 30af309..59a5675 100644
--- a/degesch.c
+++ b/degesch.c
@@ -5325,9 +5325,13 @@ mode_processor_do_user (struct mode_processor *self)
 	 || !(channel_user = irc_channel_get_user (self->channel, user)))
 		return;
 
+	// Translate mode character to user prefix character
 	const char *all_prefixes = self->s->irc_chanuser_prefixes;
 	const char *all_modes    = self->s->irc_chanuser_modes;
-	char prefix = all_prefixes[strchr (all_modes, self->mode_char) - all_modes];
+
+	const char *mode = strchr (all_modes, self->mode_char);
+	hard_assert (mode && (size_t) (mode - all_modes) < strlen (all_prefixes));
+	char prefix = all_prefixes[mode - all_modes];
 
 	struct str *prefixes = &channel_user->prefixes;
 	const char *pos = strchr (prefixes->str, prefix);