"Cannot read property 'url' of undefined" error when using a scroll ref as master in NormalizedCollection

[mmta]

This http://plnkr.co/edit/MtMBcfQAbXn64inYHaC6

Throws an Uncaught TypeError: Cannot read property 'url' of undefined warning and error in the console whenever I click the "Scroll 1" button.

I'm not sure if this is an issue with the library or the way I'm using it as in the plunker example, but other than those warning & error, the data is received in correct descending order as intended.

Here's the data export for the above plunker:

{
  "data" : {
    "T1" : {
      "field1" : "ABC1",
      "field2" : "ABC2"
    },
    "T2" : {
      "field1" : "DEF1",
      "field2" : "DEF2"
    },
    "T3" : {
      "field1" : "GHI1",
      "field2" : "GHI2"
    },
    "T4" : {
      "field1" : "JKL1",
      "field2" : "JKL2"
    }
  },
  "index" : {
    "T1" : -1,
    "T2" : -2,
    "T3" : -3,
    "T4" : -4
  }
}

Thanks!