ERROR 1366 (HY000): Incorrect string value:MySQL数据库、表的字符集为GBK

mysql> update userinfo set MEDIASOURCE = 'CS02-北京' where IMPORTNO = 'IMP201640613101206';
ERROR 1366 (HY000): Incorrect string value: '\x91' for column 'MEDIASOURCE' at row 1

mysql> show variables like '%char%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | gbk |
| character_set_connection | gbk |
| character_set_database | gbk |
| character_set_filesystem | binary |
| character_set_results | gbk |
| character_set_server | gbk |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)

mysql> set character_set_client =utf8;
Query OK, 0 rows affected (0.00 sec)

mysql> set character_set_server =utf8;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%char%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8 |
| character_set_connection | gbk |
| character_set_database | gbk |
| character_set_filesystem | binary |
| character_set_results | gbk |
| character_set_server | utf8 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)

mysql> update userinfo set MEDIASOURCE = 'CS02-北京' where IMPORTNO = 'IMP201640613101206';
Query OK, 169 rows affected (0.03 sec)
Rows matched: 169 Changed: 169 Warnings: 0