diff --git a/20230125142000_functions.up.sql b/20230125142000_functions.up.sql
index 416df33..0efddfb 100644
--- a/20230125142000_functions.up.sql
+++ b/20230125142000_functions.up.sql
@@ -1,13 +1,21 @@
 BEGIN;
 
-CREATE FUNCTION is_name(value TEXT) RETURNS BOOLEAN LANGUAGE plpgsql
+CREATE FUNCTION is_name(value TEXT)
+    RETURNS BOOLEAN
+    IMMUTABLE
+    PARALLEL SAFE
+    LANGUAGE plpgsql
 AS $$BEGIN
-    RETURN value = btrim(value) AND length(value) > 0;
+    RETURN value IS NOT NULL AND value = btrim(value) AND length(value) > 0;
 END$$;
 
-CREATE FUNCTION is_language_code(value VARCHAR) RETURNS BOOLEAN LANGUAGE plpgsql
+CREATE FUNCTION is_language_code(value VARCHAR)
+    RETURNS BOOLEAN
+    IMMUTABLE
+    PARALLEL SAFE
+    LANGUAGE plpgsql
 AS $$BEGIN
-    RETURN value ~ '^[a-z]{2}$';
+    RETURN value IS NOT NULL AND value ~ '^[a-z]{2}$';
 END$$;
 
 COMMIT;