Cannot set read-only property ReflectionObject

I'm having difficulty writing to class properties using the Reflection API. $p->setValue($r, $value) throws an exception saying that the property is read only, however, PHP (to my knowledge) has no read only class properties (only methods using final), so what's the crack here?

The problem code:

    public function apply($source, $target) {
        $r = new \ReflectionObject($target);
        foreach ($source as $key => $value) {
            $p = $r->getProperty($this->entries[$key]);
            $p->setAccessible(true);
            $p->setValue($r, $value); // <--- problem here
        }
    }

The exception reads as follows:

Uncaught ReflectionException: Cannot set read-only property ReflectionObject::$name

An excerpt from the class being reflected ($target in the above snippet), this issue remains whether I set the properties to public or keep private

    private $id;
    private $name;

Nothing special going on with this class, just a plain object with a default constructor.

Is there something in the docs that I've missed, or is this just PHP weirdness that I need to accept?

Using version 7.1