Cannot add or update a child row: a foreign key constraint fails

table #1: | Field | Type | Null | Key | Default | Extra | | -------- | -------------- | ----- |--- | --------|------ | | UserID | int(11) | NO | PRI | NULL | auto_increment | | Password | varchar(20) | NO | | | | | Username | varchar(25) | NO | | | | | Email | varchar(60) | NO | | | | table #2: | Field | Type | Null | Key | Default | Extra | | -------- | ------------ | ---- |---- | --------|--------------- | | UserID | int(11) | NO | MUL | | | | PostID | int(11) | NO | PRI | NULL | auto_increment | | Title | varchar(50) | NO | | | | | Summary | varchar(500) | NO | | | | Error: > com.mysql.jdbc.exceptions.MySQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails (`myapp/table2`, CONSTRAINT `table2_ibfk_1` FOREIGN KEY (`UserID`) REFERENCES `table1` (`UserID`)) What have I done wrong? I read http://www.w3schools.com/Sql/sql_foreignkey.asp and I don't see what's wrong.