why do i get this get/set error?

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:

Code:

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

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if i change the code to this...

Code:

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

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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!