Custom pointer property is read only?

Archive Python API
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Pretty sure i’m doing something wrong here, seems a bit strange to have a custom property default to read only (what would be the point in creating it if you can never set it to something other than the default?)

here’s a bare bones test script that can be run in a scene with a default cube:

import bpy
from bpy.props import CollectionProperty, PointerProperty, StringProperty

class CUSTOM_test(bpy.types.PropertyGroup):
    my_attr: StringProperty()
    
bpy.utils.register_class(CUSTOM_test)
bpy.types.Scene.my_test = CollectionProperty(type=CUSTOM_test)
bpy.types.Object.my_pointer = PointerProperty(type=CUSTOM_test)

bpy.context.scene.my_test.add()

# AttributeError:
bpy.context.active_object.my_pointer = bpy.context.scene.my_test[0]

which results in the following exception:
AttributeError: bpy_struct: attribute "my_pointer" from "Object" is read-only

So what’s the solution here? I have a custom PropertyGroup stored on the Scene that I need to be able to keep track of on individual objects.

nb: cross-posted on ba.org