JDBC错误：java.sql.SQLException: ORA-01008: 并非所有变量都已绑定「建议收藏」

public int addCourseTime(CourseTimeBean ctb){
int csh_no=ctb.getCsh_no();
int csh_id=ctb.getC_id();
String  csh_start_time=ctb.getCsh_start_time();
String csh_due_time=ctb.getCsh_due_time();
int csh_numbers=ctb.getCsh_numbers();
int csh_minutes=ctb.getCsh_minutes();
String sql="insert into course_schedule values(?,?,to_date(?,'yyyy-MM-dd hh-mi-ss'),to_date(?,'yyyy-MM-dd hh-mi-ss'),?,?)";
Connection conn=DButil.getConn();
PreparedStatement pstm=null;
int result=0;
try {
pstm=conn.prepareStatement(sql);
pstm.setInt(1,csh_no);
pstm.setInt(2, csh_id);
pstm.setString(3,csh_start_time);
pstm.setString(4,csh_due_time);
pstm.setInt(5, csh_numbers);
pstm.setInt(6, csh_minutes);
result =pstm.executeUpdate(sql);
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}finally{
DButil.free(pstm, conn);
return result;
}
}

java.sql.SQLException: ORA-01008: 并非所有变量都已绑定
 at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:134) at oracle.jdbc.ttc7.TTIoer.processError(TTIoer.java:289) at oracle.jdbc.ttc7.Oall7.receive(Oall7.java:573) at oracle.jdbc.ttc7.TTC7Protocol.doOall7(TTC7Protocol.java:1891) at oracle.jdbc.ttc7.TTC7Protocol.parseExecuteFetch(TTC7Protocol.java:1093) at oracle.jdbc.driver.OracleStatement.executeNonQuery(OracleStatement.java:2047) at oracle.jdbc.driver.OracleStatement.doExecuteOther(OracleStatement.java:1940) at oracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatement.java:2709) at oracle.jdbc.driver.OracleStatement.executeUpdate(OracleStatement.java:796) at cn.com.courseInfoDao.CourseTimeInfoDao.addCourseTime(CourseTimeInfoDao.java:60)