为什么在Go AppEngine aetest中使用datastore.PropertyList使用datastore.Put获得“无效实体类型”？

This test fails with partnermerge_test.go:22: datastore: invalid entity type

package bigdipper

import (
    "testing"

    "appengine/aetest"
    "appengine/datastore"
)

func TestCreateMigrationProposal(t *testing.T) {
    c, err := aetest.NewContext(nil)
    if err != nil {
            t.Fatal(err)
    }
    defer c.Close()
    if _, err := datastore.Put(
            c,
            datastore.NewKey(c, "ORDER", "order-id-1", 0, nil),
            datastore.PropertyList{}); err != nil {
            t.Fatal(err)
    }
}

The docs for the datastore.Put function say:

Put saves the entity src into the datastore with key k. src must be a struct pointer or implement PropertyLoadSaver; if a struct pointer then any unexported fields of that struct will be skipped. If k is an incomplete key, the returned key will be a unique key generated by the datastore.

This was somewhat confusing when trying to use this with a PropertyList as the src. A PropertyList does not implement PropertyLoadSaver, but a *PropertyList does. Adding an & before PropertyList to get a pointer to it fixes this test.

package bigdipper

import (
    "testing"

    "appengine/aetest"
    "appengine/datastore"
)

func TestCreateMigrationProposal(t *testing.T) {
    c, err := aetest.NewContext(nil)
    if err != nil {
            t.Fatal(err)
    }
    defer c.Close()
    if _, err := datastore.Put(
            c,
            datastore.NewKey(c, "ORDER", "order-id-1", 0, nil),
            &datastore.PropertyList{}); err != nil {
            t.Fatal(err)
    }
}