nil：NilClass <NoMethodError>的未定义方法'[]'

请帮助我，我在尝试执行此方法时遇到了NoMethodError

def traspuesta()
i=0
aux=nil
    for i in [email protected] do
            aux = @lt[i][0]
            @lt[i][0] = @lt[i][1]
            @lt[i][1] = aux
    end
end

完整的错误是这样的：

Undefined method '[]' for nil:NilClass <NoMethodError>
from MDListaTrip.rb:83:in 'each'
from MDListaTrip.rb:83:in 'traspuesta'
from MDListaTrip.rb:111:in '<main>'