diff --git a/test/yarp/comments_test.rb b/test/yarp/comments_test.rb
index 5c49f7b86e..c95438c891 100644
--- a/test/yarp/comments_test.rb
+++ b/test/yarp/comments_test.rb
@@ -6,7 +6,10 @@ class CommentsTest < Test::Unit::TestCase
   include ::YARP::DSL
 
   def test_comment_inline
-    assert_comment "# comment", :inline, 0..9
+    source = "# comment"
+
+    assert_comment source, :inline, 0..9
+    assert_equal [0], YARP.const_get(:Debug).newlines(source)
   end
 
   def test_comment_inline_def
diff --git a/yarp/util/yp_newline_list.c b/yarp/util/yp_newline_list.c
index ad9d99d0ab..de353acf62 100644
--- a/yarp/util/yp_newline_list.c
+++ b/yarp/util/yp_newline_list.c
@@ -30,6 +30,7 @@ yp_newline_list_append(yp_newline_list_t *list, const char *cursor) {
         if (list->offsets == NULL) return false;
     }
 
+    assert(*cursor == '\n');
     assert(cursor >= list->start);
     size_t newline_offset = (size_t) (cursor - list->start + 1);
     assert(list->size == 0 || newline_offset > list->offsets[list->size - 1]);
diff --git a/yarp/yarp.c b/yarp/yarp.c
index d559163288..acf4f86a19 100644
--- a/yarp/yarp.c
+++ b/yarp/yarp.c
@@ -5389,7 +5389,7 @@ parser_lex(yp_parser_t *parser) {
                 /* fallthrough */
                 case '\n': {
                     if (parser->heredoc_end == NULL) {
-                        yp_newline_list_append(&parser->newline_list, parser->current.end - 1);
+                        yp_newline_list_check_append(&parser->newline_list, parser->current.end - 1);
                     } else {
                         parser_flush_heredoc_end(parser);
                     }