ERROR : {"last block incomplete in decryption"}

[shikhargilhotra1985]

Hi Dear,
I am getting this error while i am executing it on my system almost as it is ( or with little required changes). Can you please tell me what is the error means ?
It says : {"last block incomplete in decryption"}

And gives the above exception on following line :
var privateKey = new PemReader(reader, pwf).ReadObject();

in BoxJWTAuth.cs page.

Let me know if it makes any sense to you. and respond in detail, if you can. I will relly appreciate that.