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If you type "fail" into line 18 and run it, you'll see why it's breaking here.  Basically @board is an instance of the entire Board class, but what you (I assume) really want to check is whether that actual game board itself ls valid.  

At this point in the code, @board = `#<Board:0x00000101ab3798 @current_board=[[nil, nil, nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil, nil, nil]]>`.

What you really want to do is access @current_board within @board.  You can do this by defining `attr_reader :current_board` inside of `board.rb` and then calling `@board.current_board` here rather than just @board.  You'll still have to tweak the methods a bit, but that will get things on the right track