Highchart network graph with tooltip when hover on every node

I am creating network graphs using highcharts for one of my Angular project. I want to create tooltip when mouse moves on to a point/node. I tried to create using below chart options. But, I am getting error. How can I create tooltip for networkgraph points?

 chartOptions = {
  chart: {
    type: "networkgraph",
    height: "100%"      
  },
  title: {
    text: "Network graph demo"
  },
  subtitle: {
    text: "A Force-Directed Network Graph in Highcharts"
  },
  plotOptions: {
    networkgraph: {
      keys: ["from", "to"],
      layoutAlgorithm: {
        enableSimulation:true
      }
    }

  },
  series: [
    {
      dataLabels: {
        enabled: true,            
        linkFormat: ''          
      },
      data: [{from : 'a', to: 'b'},
              {from: 'a', to: 'c'},
               {from: 'a', to: 'd'} ],
      marker : {radius : 18}
    }

  ],
  tooltip :
   {
    enabled : true, 
    formatter : { function() {
      return "<div> <span> My tooltip information </span> </div>"
    }}
  }
}

Error:

 ERROR TypeError: w.call is not a function
at a.Tooltip.refresh (highcharts.js:205)
at a.Pointer.runPointActions (highcharts.js:220)
at q.onMouseOver (highcharts.js:446)
at SVGGElement.d [as __zone_symbol__ON_PROPERTYmouseover] (highcharts.js:435)
at SVGGElement.wrapFn (zone.js:1332)
at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:423)
at Object.onInvokeTask (core.js:17290)
at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:422)
at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runTask (zone.js:195)
at ZoneTask.push../node_modules/zone.js/dist/zone.js.ZoneTask.invokeTask [as invoke] (zone.js:498)

tooltip seems wrong. (extra curly braces)

try this code:

 tooltip :
   {
    enabled : true, 
    formatter : function() {
      return "<div> <span> My tooltip information </span> </div>"
    }
  }