diff --git a/lib/erb/compiler.rb b/lib/erb/compiler.rb
index 547d2c4c44..7096c8dcea 100644
--- a/lib/erb/compiler.rb
+++ b/lib/erb/compiler.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 #--
 # ERB::Compiler
 #
diff --git a/lib/erb/def_method.rb b/lib/erb/def_method.rb
index 17f9c0f9fa..aee989a926 100644
--- a/lib/erb/def_method.rb
+++ b/lib/erb/def_method.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 #--
 # ERB::DefMethod
 #
diff --git a/lib/erb/util.rb b/lib/erb/util.rb
index 0c1e7482a8..1d2a36275d 100644
--- a/lib/erb/util.rb
+++ b/lib/erb/util.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 #--
 # ERB::Escape
 #