diff --git a/string.c b/string.c
index 59dd0c2c75..a8580c7860 100644
--- a/string.c
+++ b/string.c
@@ -7314,7 +7314,8 @@ mapping_buffer_free(void *p)
 
 static const rb_data_type_t mapping_buffer_type = {
     "mapping_buffer",
-    {0, mapping_buffer_free,}
+    {0, mapping_buffer_free,},
+    0, 0, RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED
 };
 
 static VALUE