why do i get this get/set error?

[PT Exorcist]

Cannot modify the return value of 'Branco.Irc.IrcUserCollection.this[string]' because it is not a variable

this error happens when i have the following code:

_channels[""].Users[""].UserName = "";

if i change the code to this...

IrcUser c = _channels[""].Users[""];
c.UserName = "";

then i dont get any error so it has something to do with the get/set..but that propertys have both the get/set so i dont get where might be the problem :S

any help plz!