NBitcoin throws InvalidOperationException with the message: "Mac HMACSHA256 not recognised."

I dig into the opensource NBitcoin API and figured out these lines give me the error message. What can I do? (http://ift.tt/1HG6VV8)

try
            {
                hmac = MacUtilities.GetMac(macName);
            }
            catch(SecurityUtilityException nsae)
            {
                throw new InvalidOperationException(nsae.Message, nsae);
            }