beam.Create fails to encode valid elements

package main

import (
	"context"

	"github.com/apache/beam/sdks/go/pkg/beam"
	"github.com/apache/beam/sdks/go/pkg/beam/x/beamx"
	"github.com/apache/beam/sdks/go/pkg/beam/x/debug"

	ipb "kythe.io/kythe/proto/internal_go_proto"
	srvpb "kythe.io/kythe/proto/serving_go_proto"
)

func main() {
	ctx := context.Background()
	beam.Init()

	p := beam.NewPipeline()
	s := p.Root()

	e := &ipb.Path_Node{
		// proto3 oneof fields cannot be JSON marshalled
		Specialization: &ipb.Path_Node_RawAnchor{&srvpb.RawAnchor{}},
	}
	els := beam.Create(s, e)
	debug.Print(s, els)

	if err := beamx.Run(ctx, p); err != nil {
		panic(err)
	}
}