Serialize Map<string, string option> raises "Object reference not set to an instance of an object."

[codingedgar]

Hello! Currently, I'm trying to serialize this scenario but it raises an error:

type Example = Map<string, string option>

let example: Example = Map.empty

example
|> Map.add "p" None
|> Json.serialize // "Object reference not set to an instance of an object."

When I step with the debugger I see: I guess F# interprets a value of type option as null in maps and it throws?