C#, NBitcoin and XPubkey

I'm attempting to write an algorithm that generates public bitcoin addresses from a known xpubkey. The key I'm using for testing can be found at blockchain.info at

https://blockchain.info/xpub/xpub6CUGRUonZSQ4TWtTMmzXdrXDtypWKiKrhko4egpiMZbpiaQL2jkwSB1icqYh2cfDfVxdx4df189oLKnC5fSwqPfgyP3hooxujYzAu3fDVmz

I'm trying something like this to generate the address on the path 0,0,0, as below:

Code:

var pubkey = new ExtPubKey(Encoding.ASCII.GetBytes("6CUGRUonZSQ4TWtTMmzXdrXDtypWKiKrhko4egpiMZbpiaQL2jkwSB1icqYh2cfDfVxdx4df189oLKnC5fSwqPfgyP3hooxujYzAu3fDVmz"));
Console.Writeline(pubkey.Derive(0).Derive(0).Derive(0).PubKey.GetAddress(Network.Main));

I'm not fully understanding something because I get a 'Invalid point encoding 103' when trying to instantiate the ExtPubKey. Does anyone have working sample code?