the lexer attempts to error with Issues::HEREDOC_MISSING_ENDTAG. Since there's no such issue, this instead results in a NameError. The correct issue to use here, given that there's no actual tag, would be Issues::HEREDOC_EMPTY_TAG.
If a heredoc used an empty endtag @("") it did not error the correct way and would report a Ruby NameError. Now the error correctly states that the error is raised because the end tag is empty.