diff --git a/app/validators/not_nil_validator.rb b/app/validators/not_nil_validator.rb
index ac97716dbd8..dcb0db286f0 100644
--- a/app/validators/not_nil_validator.rb
+++ b/app/validators/not_nil_validator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #-- copyright
 # OpenProject is an open source project management software.
 # Copyright (C) the OpenProject GmbH
diff --git a/app/validators/secure_context_uri_validator.rb b/app/validators/secure_context_uri_validator.rb
index 573beec7f29..3c7e2f921aa 100644
--- a/app/validators/secure_context_uri_validator.rb
+++ b/app/validators/secure_context_uri_validator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #-- copyright
 # OpenProject is an open source project management software.
 # Copyright (C) the OpenProject GmbH
diff --git a/app/validators/url_validator.rb b/app/validators/url_validator.rb
index 939f6132387..ca9e8171700 100644
--- a/app/validators/url_validator.rb
+++ b/app/validators/url_validator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Borrows from http://gist.github.com/bf4/5320847
 # without addressable requirement
 # Accepts options[:allowed_protocols]