How to animate custom material property in RealityKit?

Question

I can successfully load a custom ShaderGraphMaterial material loaded from a .usda file created with Reality Composer

var shotTrailMaterial : ShaderGraphMaterial? = nil
do {
   ...
   let materialData = try Data(contentsOf: materialURL)
   shotTrailMaterial = try await ShaderGraphMaterial(named:"/Root/Material", 
                                                      from: materialData)
} catch { ... }

for which I can set the "fractionOpaque" custom property to a floating point value

try shotTrailMaterial.setParameter(name: "fractionOpaque", value: .float(0.9))

The above works well for the entities I create with this material. Now I would like to animate the material based on this parameter but the following fails:

let anim = FromToByAnimation<Float>(name: "foo",
                                    from: 0.0,
                                      to: 1.0,
                                duration: 3.0,
                              bindTarget: .parameter("fractionOpaque"))
if let animResource = try? AnimationResource.generate(with: anim) {
    entity.playAnimation(animResource, transitionDuration: 3.0, startsPaused: false)
}

I get the error

Cannot find a BindPoint for any bind path: "KeyValue.keyValueStore[fractionOpaque]"

Clearly my key path is wrong for the bindTarget: parameter. I want to set the "fractionOpaque" property on material 0, but I am not sure what the correct path for this is? This is a Vision OS app.