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Message: [QUOTE="Jacob Fugal, post: 4508511"] Take a look at this for some clues: $ irb irb(main):001:0> events =3D {} =3D> {} irb(main):002:0> events.default =3D [] =3D> [] irb(main):003:0> events["trying"] << "hello" =3D> ["hello"] irb(main):004:0> events.default =3D> ["hello"] irb(main):005:0> events["test"] =3D> ["hello"] In short, what's happening is that when you call events["trying"] at statement 3, it returns the default value, which is an array. You then insert "hello" into that array and *change the default value*. Since you never assigned to events["trying"], it still doesn't exist. You were only getting the expected value back on re-access because of the changed default value. Try this instead: $ irb irb(main):001:0> events =3D Hash.new{ |h,k| h[k] =3D [] } =3D> {} irb(main):002:0> events.default =3D> [] irb(main):003:0> events["trying"] << "hello" =3D> ["hello"] irb(main):004:0> events.default =3D> [] irb(main):005:0> events["trying"] =3D> ["hello"] irb(main):006:0> events["test"] =3D> [] irb(main):007:0> events =3D> {nil=3D>[], "test"=3D>[], "trying"=3D>["hello"]} This produces the desired behavior, but you have to be careful, since accessing non-existent keys will pollute the keys of your hash. [/QUOTE]