diff --git a/src/filter.c b/src/filter.c
index 7acc0d9..8502d78 100644
--- a/src/filter.c
+++ b/src/filter.c
@@ -2650,12 +2650,14 @@ filter_user_get(filter_t *filter, filter_rule_t *rule)
 }
 #endif /* WITHOUT_USERAUTH */
 
+/*
+ * Translates filtering rules into data structures.
+ * Never pass NULL as rule param.
+ * Otherwise, we must return NULL, but NULL retval means oom.
+ */
 filter_t *
 filter_set(filter_rule_t *rule)
 {
-	if (!rule)
-		return NULL;
-
 	filter_t *filter = malloc(sizeof(filter_t));
 	if (!filter)
 		return oom_return_na_null();
diff --git a/src/filter.h b/src/filter.h
index 98b694a..7233cc7 100644
--- a/src/filter.h
+++ b/src/filter.h
@@ -285,7 +285,7 @@ filter_keyword_t *filter_keyword_find(filter_t *, filter_user_t *, char *) NONNU
 filter_user_t *filter_user_find(filter_t *, char *) NONNULL(1,2);
 #endif /* !WITHOUT_USERAUTH */
 int filter_rule_set(opts_t *, const char *, char *, int) NONNULL(1,2,3) WUNRES;
-filter_t *filter_set(filter_rule_t *);
+filter_t *filter_set(filter_rule_t *) NONNULL(1) WUNRES;
 
 #endif /* !FILTER_H */