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https://heycam.github.io/webidl/#dfn-attribute-setter [[ Let I be the interface whose interface prototype object this property corresponding to the attribute appears on. ]] In the case of a global object, there is no such IPO.
Nor in the case of an [Unforgeable] attribute, right?
Fixed in https://github.com/heycam/webidl/commit/4c9c298