Why error when add 'source' attribute to data?

myDiagram.nodeTemplateMap.add("",  // the default category
        $(go.Node, "Table", nodeStyle(),
          // the main object is a Panel that surrounds a TextBlock with a rectangular Shape
          $(go.Panel, "Auto",
            $(go.Shape, "Rectangle",
              { fill: $(go.Brush, "Linear", { 0.0: "rgb(172,192,242)", 1.0: "rgb(124,134,221)" }), strokeWidth: 0, minSize: new go.Size(100, 80)},
              new go.Binding("figure", "figure")),
            $(go.Panel, "Horizontal",
                { margin: 10, width: 50, height: 50, background: "red" },
                new go.Binding("source")),
              $(go.TextBlock, textStyle(),
                  margin: 8,
                  maxSize: new go.Size(160, NaN),
                  wrap: go.TextBlock.WrapFit,
                  editable: true
                new go.Binding("text").makeTwoWay())
          // four named ports, one on each side:
          makePort("T", go.Spot.Top, go.Spot.TopSide, false, true),
          makePort("L", go.Spot.Left, go.Spot.LeftSide, true, true),
          makePort("R", go.Spot.Right, go.Spot.RightSide, true, true),
          makePort("B", go.Spot.Bottom, go.Spot.BottomSide, true, false)
        ));`Preformatted text`

why error when add ‘source’ attribute to data?

{"text":"步骤", "key":-4, "loc":"-192 131", source:"cat1.png"}

go.js:1736 Uncaught TypeError: Cannot read property ‘class’ of null
at Function.Z.fromJSON.Z.fromJson (go.js:1736)
at load (flowchart.html:298)
at init (flowchart.html:275)
at onload (flowchart.html:322)

That is invalid syntax JSON. All property names must be quoted with double quotes.