Application Error

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'user_id' cannot be null, query was: insert into user_follow_author (author_id,user_id,is_follow,created)values (?,?,?,?)
          on duplicate key update is_follow=?
#0 /data/wap/application/library/Zend/Db/Statement.php(303): Zend_Db_Statement_Pdo->_execute(Array)
#1 /data/wap/application/library/Zend/Db/Adapter/Abstract.php(480): Zend_Db_Statement->execute(Array)
#2 /data/wap/application/library/Zend/Db/Adapter/Pdo/Abstract.php(238): Zend_Db_Adapter_Abstract->query('insert into use...', Array)
#3 /data/wap/application/models/Author.php(291): Zend_Db_Adapter_Pdo_Abstract->query('insert into use...', Array)
#4 /data/wap/application/models/Svr/Article.php(41): AuthorModel->makeFollow(NULL, '2', 1)
#5 /data/wap/application/modules/Pc/controllers/Article.php(358): Svr_ArticleModel->addVirtualFocus(10613)
#6 [internal function]: ArticleController->detailAction()
#7 /data/wap/public/index.php(52): Yaf_Application->run()
#8 {main}