ECS service capacity provider: using custom-resources getResponseField and parseInt, ends up with value set to null

[altonotch]

Using following code:

const cpStrategy: ecs.CapacityProviderStrategy = {
  capacityProvider: clusterDetails.getResponseField(
    "clusters.0.defaultCapacityProviderStrategy.0.capacityProvider",
  ),
  base: parseInt(
    clusterDetails.getResponseField(
      "clusters.0.defaultCapacityProviderStrategy.0.base",
    ),
  ),
  weight: parseInt(
    clusterDetails.getResponseField(
      "clusters.0.defaultCapacityProviderStrategy.0.weight",
    ),
  ),
};

Which translates to this cloud formation template:

"CapacityProviderStrategy": [{
  "Base": null,
  "CapacityProvider": {
   "Fn::GetAtt": [
    "ClusterDetails9A67C6F8",
    "clusters.0.defaultCapacityProviderStrategy.0.capacityProvider"
   ]
  },
  "Weight": null
 }
]

Both Base and Weight are null.
Is this normal behaviour or some kind of a bug?