Base64解码＆＃34;输入无效Base64字符串＆＃34;

Question

为什么我无法使用此代码解码Base64？它说"Input not valid Base64 string, because it contains characters that are not the Base-64 encoding, or more than two fill characters or contains invalid characters between fillings."

我的代码是

    Dim base64_Encoded As String = "YXNkZg=="
    Dim bt64 As Byte() = System.Convert.FromBase64String(base64_Encoded)***
    Dim text As String = System.Text.Encoding.Unicode.GetString(bt64)
    MsgBox(text)

其中YXNkZg== = asdf和***是卡住的地方。