Array#split behaviour when accessing beyond the array boundry

An irb session is following:

>> ar = [:peanute, :butter, :and, :jelly]
=> [:p开发者_开发问答eanute, :butter, :and, :jelly]
>> ar[0, 1]
=> [:peanute]
>> ar[4, 1]
=> []
>> ar[5, 1]
=> nil

Why ar[4, 1] does not return nil just like ar[5, 1]? Would someone explain this behaviour?

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This is mentioned in the documentation as a special case.