diff --git a/ruby/ext/google/protobuf_c/convert.c b/ruby/ext/google/protobuf_c/convert.c
index c9448cd..b97c050 100644
--- a/ruby/ext/google/protobuf_c/convert.c
+++ b/ruby/ext/google/protobuf_c/convert.c
@@ -141,7 +141,7 @@
       VALUE utf8 = rb_enc_from_encoding(rb_utf8_encoding());
       if (rb_obj_class(value) == rb_cSymbol) {
         value = rb_funcall(value, rb_intern("to_s"), 0);
-      } else if (rb_obj_class(value) != rb_cString) {
+      } else if (!rb_obj_is_kind_of(value, rb_cString)) {
         rb_raise(cTypeError,
                  "Invalid argument for string field '%s' (given %s).", name,
                  rb_class2name(CLASS_OF(value)));
diff --git a/ruby/tests/basic.rb b/ruby/tests/basic.rb
index fe2119e..1e5e7a3 100755
--- a/ruby/tests/basic.rb
+++ b/ruby/tests/basic.rb
@@ -787,4 +787,15 @@
     assert_respond_to msg, :has_d?
     refute msg.has_d?
   end
+
+  def test_string_subclass
+    str = "hello"
+    myString = Class.new(String)
+
+    m = proto_module::TestMessage.new(
+      optional_string: myString.new(str),
+    )
+
+    assert_equal str, m.optional_string
+  end
 end