diff --git a/sql/mysql_priv.h b/sql/mysql_priv.h
index a0f34434254285b202b81845f6b02ae1f01c8d83..721e032a5b094053f7844837fd554728205a1a21 100644
--- a/sql/mysql_priv.h
+++ b/sql/mysql_priv.h
@@ -807,6 +807,7 @@ extern SHOW_COMP_OPTION have_berkeley_db;
 extern struct system_variables global_system_variables;
 extern struct system_variables max_system_variables;
 extern struct rand_struct sql_rand;
+extern String null_string;
 extern HASH open_cache;
 extern TABLE *unused_tables;
 extern I_List<i_string> binlog_do_db, binlog_ignore_db;