diff --git a/lib/erb.rb b/lib/erb.rb
index 445d479..1bc6da6 100644
--- a/lib/erb.rb
+++ b/lib/erb.rb
@@ -833,6 +833,7 @@ def initialize(str, trim_mode: nil, eoutvar: '_erbout')
     compiler = make_compiler(trim_mode)
     set_eoutvar(compiler, eoutvar)
     @src, @encoding, @frozen_string = *compiler.compile(str)
+    @src.freeze
     @filename = nil
     @lineno = 0
     @_init = self.class.singleton_class
diff --git a/test/erb/test_erb.rb b/test/erb/test_erb.rb
index 09496d3..f6133b1 100644
--- a/test/erb/test_erb.rb
+++ b/test/erb/test_erb.rb
@@ -598,10 +598,10 @@ def stags
   def test_frozen_string_literal
     bug12031 = '[ruby-core:73561] [Bug #12031]'
     e = @erb.new("<%#encoding: us-ascii%>a")
-    e.src.sub!(/\A#(?:-\*-)?(.*)(?:-\*-)?/) {
+    src = e.src.sub(/\A#(?:-\*-)?(.*)(?:-\*-)?/) {
       '# -*- \1; frozen-string-literal: true -*-'
     }
-    assert_equal("a", e.result, bug12031)
+    assert_equal("a", eval(src), bug12031)
 
     %w(false true).each do |flag|
       erb = @erb.new("<%#frozen-string-literal: #{flag}%><%=''.frozen?%>")