Assertion `fixed == 1' failed in virtual String* Item_func_user::val_str with PS

SET NAMES latin1;

PREPARE stmt FROM "SELECT NULLIF(CURRENT_USER, @A := 'foo')";

EXECUTE stmt;

mysqld: /data/src/10.1/sql/item_strfunc.h:670: virtual String* Item_func_user::val_str(String*): Assertion `fixed == 1' failed.

170317 20:24:24 [ERROR] mysqld got signal 6 ;

#7  0x00007f6c573a2312 in __GI___assert_fail (assertion=0x7f6c5a58d684 "fixed == 1", file=0x7f6c5a58d578 "/data/src/10.1/sql/item_strfunc.h", line=670, function=0x7f6c5a58f720 <Item_func_user::val_str(String*)::__PRETTY_FUNCTION__> "virtual String* Item_func_user::val_str(String*)") at assert.c:101

#8  0x00007f6c59e83c87 in Item_func_user::val_str (this=0x7f6c4e5a75b8) at /data/src/10.1/sql/item_strfunc.h:670

#9  0x00007f6c59e21147 in Item_func_nullif::str_op (this=0x7f6c4e5a7c10, str=0x7f6c4e5a7c40) at /data/src/10.1/sql/item_cmpfunc.cc:2883

#10 0x00007f6c59e5f8d2 in Item_func_hybrid_field_type::str_op_with_null_check (this=0x7f6c4e5a7c10, str=0x7f6c4e5a7c40) at /data/src/10.1/sql/item_func.h:441

#11 0x00007f6c59e4aeec in Item_func_hybrid_field_type::val_str (this=0x7f6c4e5a7c10, str=0x7f6c5aab7e80) at /data/src/10.1/sql/item_func.cc:873

#12 0x00007f6c59e082ff in Item::send (this=0x7f6c4e5a7c10, protocol=0x7f6c503fa600, buffer=0x7f6c5aab7e80) at /data/src/10.1/sql/item.cc:6521

#13 0x00007f6c59af597e in Protocol::send_result_set_row (this=0x7f6c503fa600, row_items=0x7f6c4e5a6970) at /data/src/10.1/sql/protocol.cc:914

#14 0x00007f6c59b6d4d8 in select_send::send_data (this=0x7f6c4e5a7e20, items=...) at /data/src/10.1/sql/sql_class.cc:2830

#15 0x00007f6c59bed9ce in JOIN::exec_inner (this=0x7f6c4e443230) at /data/src/10.1/sql/sql_select.cc:2590

#16 0x00007f6c59bed44b in JOIN::exec (this=0x7f6c4e443230) at /data/src/10.1/sql/sql_select.cc:2511

#17 0x00007f6c59bf09c9 in mysql_select (thd=0x7f6c503fa070, rref_pointer_array=0x7f6c4e5a6ad0, tables=0x0, wild_num=0, fields=..., conds=0x0, og_num=0, order=0x0, group=0x0, having=0x0, proc_param=0x0, select_options=2416184064, result=0x7f6c4e5a7e20, unit=0x7f6c4e5a6158, select_lex=0x7f6c4e5a6858) at /data/src/10.1/sql/sql_select.cc:3445

#18 0x00007f6c59be6422 in handle_select (thd=0x7f6c503fa070, lex=0x7f6c4e5a6090, result=0x7f6c4e5a7e20, setup_tables_done_option=0) at /data/src/10.1/sql/sql_select.cc:384

#19 0x00007f6c59bb6387 in execute_sqlcom_select (thd=0x7f6c503fa070, all_tables=0x0) at /data/src/10.1/sql/sql_parse.cc:5920

#20 0x00007f6c59bac883 in mysql_execute_command (thd=0x7f6c503fa070) at /data/src/10.1/sql/sql_parse.cc:2979

#21 0x00007f6c59bd3f68 in Prepared_statement::execute (this=0x7f6c4e585470, expanded_query=0x7f6c5aab9a10, open_cursor=false) at /data/src/10.1/sql/sql_prepare.cc:4299

#22 0x00007f6c59bd2dda in Prepared_statement::execute_loop (this=0x7f6c4e585470, expanded_query=0x7f6c5aab9a10, open_cursor=false, packet=0x0, packet_end=0x0) at /data/src/10.1/sql/sql_prepare.cc:3931

#23 0x00007f6c59bd0e3d in mysql_sql_stmt_execute (thd=0x7f6c503fa070) at /data/src/10.1/sql/sql_prepare.cc:3055

#24 0x00007f6c59bac8b4 in mysql_execute_command (thd=0x7f6c503fa070) at /data/src/10.1/sql/sql_parse.cc:2990

#25 0x00007f6c59bb9aa5 in mysql_parse (thd=0x7f6c503fa070, rawbuf=0x7f6c4e443088 "EXECUTE stmt", length=12, parser_state=0x7f6c5aaba5e0) at /data/src/10.1/sql/sql_parse.cc:7343

#26 0x00007f6c59ba8a92 in dispatch_command (command=COM_QUERY, thd=0x7f6c503fa070, packet=0x7f6c51d3e071 "", packet_length=12) at /data/src/10.1/sql/sql_parse.cc:1490

#27 0x00007f6c59ba77f1 in do_command (thd=0x7f6c503fa070) at /data/src/10.1/sql/sql_parse.cc:1109

#28 0x00007f6c59cde342 in do_handle_one_connection (thd_arg=0x7f6c503fa070) at /data/src/10.1/sql/sql_connect.cc:1349

#29 0x00007f6c59cde0a6 in handle_one_connection (arg=0x7f6c503fa070) at /data/src/10.1/sql/sql_connect.cc:1261

#30 0x00007f6c5a093c3c in pfs_spawn_thread (arg=0x7f6c56827ef0) at /data/src/10.1/storage/perfschema/pfs.cc:1860

#31 0x00007f6c592a4064 in start_thread (arg=0x7f6c5aabbb00) at pthread_create.c:309

#32 0x00007f6c5745c62d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111