(null)
You are seeing this because your music is not "tagged" correctly. Not the tags you can affix to a file here on last.fm, but rather the tags that can be edited within the file itself. Use whatever media player you use to look at "File information" (or something similar) and fill in the correct artist and track title information at the very least. It is also possible that you have scrobbled a file which can not contain this kind of data.
Read more about (null) on Last.fm.
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