求助关于 base64 的问题

Base64 报错

public static final String JWT_SECRET = "123";

String stringKey = Constant.JWT_SECRET;

byte[] encodedKey = Base64.decodeBase64(stringKey);

代码报错

java.lang.IllegalArgumentException: Last encoded character (before the paddings if any) is a valid base 64 alphabet but not a possible value. Expected the discarded bits to be zero.

修改后

public static final String str"1234;

String stringKey = Constant.str;

byte[] encodedKey = Base64.decodeBase64(stringKey);

运行成功
这是什么原因呢？