diff --git a/ext/json/ext/parser/parser.c b/ext/json/ext/parser/parser.c
index 70e34edb7..b3b6bfdca 100644
--- a/ext/json/ext/parser/parser.c
+++ b/ext/json/ext/parser/parser.c
@@ -2113,9 +2113,7 @@ static VALUE cParser_parse(JSON_ParserConfig *config, VALUE src)
     // Ensure the string isn't mutated under us.
     // The classic API to use is `rb_str_locktmp`, but then we'd
     // need to use `rb_protect` to make sure we always unlock.
-    if (Vsource == src) {
-        Vsource = rb_str_new_frozen(Vsource);
-    }
+    Vsource = rb_str_new_frozen(Vsource);
 
     VALUE rvalue_stack_buffer[RVALUE_STACK_INITIAL_CAPA];
     rvalue_stack value_stack = {
diff --git a/test/json/json_parser_test.rb b/test/json/json_parser_test.rb
index d9e3256bd..0b9d5fc96 100644
--- a/test/json/json_parser_test.rb
+++ b/test/json/json_parser_test.rb
@@ -1045,6 +1045,41 @@ def test_mutating_source_string_during_parsing
     assert_equal expected, actual
   end
 
+  def test_adversary_encode
+    fake_string = Class.new(String) do
+      def encode(*)
+        42
+      end
+    end
+
+    assert_raise TypeError, Encoding::InvalidByteSequenceError do
+      JSON.parse(fake_string.new("\xff".b, encoding: Encoding::UTF_16))
+    end
+
+    if RUBY_ENGINE != "jruby"
+      fake_string = Class.new(String) do
+        attr_reader :encoded_str
+
+        def initialize(...)
+          super
+          @encoded_str = JSON.dump(1000.times.to_a)
+        end
+
+        def encode(*)
+          @encoded_str
+        end
+      end
+
+      fake = fake_string.new("blah", encoding: Encoding::UTF_16)
+      assert_equal 1000.times.to_a, JSON.parse(fake, on_load: ->(v) {
+        if v == 20
+          fake.encoded_str.clear
+        end
+        v
+      })
+    end
+  end
+
   private
 
   def assert_equal_float(expected, actual, delta = 1e-2)