setComponentPropertyReferences() fails: Could not create a new component property reference

In a plugin, I’ve got code that’s trying to apply and instance swap property to particular node in a component.

The code is 

   const prop = Object.keys(this.componentSet().componentPropertyDefinitions).find((key) => key.startsWith("Peek view#"));
    if (prop) {
      try {
        const newRefs = {
          mainComponent: prop,
        };
        console.log(newRefs);
        peekView.componentPropertyReferences = newRefs;
      } catch (error) {
        console.error(`Failed to set peek view property reference for ${prop}:`, error);
      }
    }

This generates a log message showing that (I think!) the new property references are correct:

{
    "mainComponent": "Peek view#208:0"
}

But the assignment fails and throws this exception:

{
    "message": "in set_componentPropertyReferences: Could not create a new component property reference."
}