diff --git a/test/irb/test_ruby-lex.rb b/test/irb/test_ruby-lex.rb
index f7d331bc24..b5255e9b53 100644
--- a/test/irb/test_ruby-lex.rb
+++ b/test/irb/test_ruby-lex.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: false
 require 'test/unit'
 require 'irb/ruby-lex'
+require 'stringio'
 
 module TestIRB
   class TestRubyLex < Test::Unit::TestCase
@@ -10,9 +11,28 @@ module TestIRB
 
     def test_set_input_proc
       called = false
-      @scanner.set_input(self) {|x| called = true; nil}
-      assert_nil(@scanner.lex)
+      @scanner.set_input(nil) {called = true; nil}
+      @scanner.each_top_level_statement {}
       assert(called)
     end
+
+    def test_comment
+      assert_equal([["#\n", 1]], top_level_statement("#\n"))
+    end
+
+    def top_level_statement(lines)
+      input = InputLines.new(lines, "r")
+      scanned = []
+      @scanner.set_input(input)
+      @scanner.each_top_level_statement {|*e|
+        scanned << e
+        yield(*e) if defined?(yield)
+      }
+      scanned
+    end
+
+    class InputLines < StringIO
+      alias encoding external_encoding
+    end
   end
 end