Function frame_of 

fn frame_of<T: Serialize>(value: &T, session: Option<&str>) -> Value

One message as the page carries it, or the notice that says it will not.

A failed encode used to cross as Value::Null, and the fold reads type off every frame - so the page threw on exactly the frame it could not describe. The notice names the failure and draws in its place: nothing is dropped, and nothing arrives as a value no reader can read. The warning names the session the frame belongs to, which is the identity a reader of the log can act on.