diff --git a/include/ruby/ruby.h b/include/ruby/ruby.h
index 95965d6b6a8713..60fc1a5724bbf6 100644
--- a/include/ruby/ruby.h
+++ b/include/ruby/ruby.h
@@ -833,6 +833,7 @@ struct RString {
 };
 #define RSTRING_NOEMBED FL_USER1
 #define RSTRING_FSTR FL_USER17
+#define RSTRING_HTML_SAFE FL_USER18
 #define RSTRING_EMBED_LEN_MASK (FL_USER2|FL_USER3|FL_USER4|FL_USER5|FL_USER6)
 #define RSTRING_EMBED_LEN_SHIFT (FL_USHIFT+2)
 #define RSTRING_EMBED_LEN(str) \