associations - Using Datasets in Mathematica 10.0

Bug introduced in 10.0 and fixed in 10.1

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I am trying to use the Dataset functionality which was introduced in Mathematica 10.0 :

   dataset = Dataset[{
      <|"a" -> 1, "b" -> "x", "c" -> {1}|>,
      <|"a" -> 2, "b" -> "y", "c" -> {2, 3}|>,
      <|"a" -> 3, "b" -> "z", "c" -> {3}|>,
      <|"a" -> 4, "b" -> "x", "c" -> {4, 5}|>,

      <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>,
      <|"a" -> 6, "b" -> "z", "c" -> {}|>}]

But this gives me an error as show in the image below:

I tried the same syntax in combination with

Needs["TypeSystem`"];

but with the same result. How can I resolve this issue?