Cloudformation ToJSONString error in Cloudwatch DashboardBody InsufficientCapabilitiesException

I'm trying to use ToJsonString in Cloudformation for a Cloudwatch Dashboard. I get the following error:

{ status: 400,
         message:
          'Error: BotoServerError: 400 Bad Request
    {"Error": {
         "Code":"InsufficientCapabilitiesException",
         "Message":"Requires capabilities : [CAPABILITY_AUTO_EXPAND]",
         "Type":"Sender"
    },
    "RequestId":"c740f653-8a51-4ef7-bf7b-d8e94e78ee8e"}' 
}

Here is the template:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Transform": "AWS::LanguageExtensions",
  "Description": "PES Dashboard",
  "Parameters": {
    "Environment": {
      "Description": "The name of the environment. test or live",
      "Type": "String",
      "AllowedValues": [
        "test",
        "live"
      ]
    }
  },
  "Resources": {
    "HealthMonitor": {
      "Type": "AWS::CloudWatch::Dashboard",
      "Properties": {
        "DashboardBody": {
          "Fn::ToJsonString": {
            "widgets": [
              {
                "type": "metric",
                "x": 0,
                "y": 0,
                "width": 6,
                "height": 6,
                "properties": {
                  "metrics": [
                    [
                      "TPC/PES/Wevs",
                      "ResolveOrgUnit",
                      "Environment",
                      {
                        "Ref": "Environment"
                      },
                      {
                        "id": "resolved",
                        "label": "resolved",
                        "color": "#2ca02c"
                      }
                    ],
                    [
                      {
                        "expression": "total-resolved",
                        "label": "unresolved",
                        "id": "unresolved",
                        "region": "eu-west-1",
                        "color": "#d62728"
                      }
                    ],
                    [
                      "TPC/PES/Wevs",
                      "ResolveOrgUnit",
                      "Environment",
                      {
                        "Ref": "Environment"
                      },
                      {
                        "id": "total",
                        "stat": "SampleCount",
                        "visible": false,
                        "color": "#1f77b4"
                      }
                    ]
                  ],
                  "view": "pie",
                  "region": "eu-west-1",
                  "period": 86400,
                  "stat": "Sum",
                  "title": "Resolved OrgUnits"
                }
              },
              {
                "type": "metric",
                "x": 6,
                "y": 0,
                "width": 6,
                "height": 6,
                "properties": {
                  "metrics": [
                    [
                      "TPC/PES/Wevs",
                      "ResolveHome",
                      "Environment",
                      {
                        "Ref": "Environment"
                      },
                      {
                        "id": "resolved",
                        "stat": "Sum",
                        "label": "Resolved",
                        "color": "#2ca02c"
                      }
                    ],
                    [
                      {
                        "expression": "total-resolved",
                        "label": "unresolved",
                        "id": "unresolved",
                        "region": "eu-west-1",
                        "color": "#d62728",
                        "period": 86400
                      }
                    ],
                    [
                      "TPC/PES/Wevs",
                      "ResolveHome",
                      "Environment",
                      {
                        "Ref": "Environment"
                      },
                      {
                        "id": "total",
                        "visible": false,
                        "color": "#1f77b4"
                      }
                    ]
                  ],
                  "view": "pie",
                  "region": "eu-west-1",
                  "period": 86400,
                  "stat": "SampleCount",
                  "title": "Resolved Homes"
                }
              }
            ]
          }
        },
        "DashboardName": {
          "Fn::Sub": "${Environment}-pes-dashboard"
        }
      }
    }
  }
}

Can't find anything in the docs to assist, and I'm stuck for what to try next or if this is just not possible.