无法将char转换为整数

Question

我有一个变量FACTOR，观察到的是“ FAC_11”，我正在尝试以下操作：

select distinct factor from sp500_2003 
       where CAST(CAST(substr(factor,5,1) AS CHAR)AS INT)  = 1;

我收到错误消息：

Invalid character string format for type INTEGER.

尽管

select distinct factor from sp500_2003 where CAST(substr(factor,5,1) AS CHAR)  = '1';

文档说我们应该能够将CHAR强制转换为INT。