Creating JSON bodies, using anonymous F# records, for API requests

JsonRecord.fs

type Request = {
    ApiToken: ApiToken
    ProjectId: ProjectId
    StartingAt: ObjectId option
}

module Request =
    let toJson request =
        let (ProjectId projectId) = request.ProjectId
        let baseJson = {| scope = {| project = projectId
                                     recurse = true |}
                          describe = true |}

        match request.StartingAt with
            | Some (ObjectId objectId) ->
                {| baseJson with starting = {| project = projectId
                                               id = objectId |} |}

            | None ->
                baseJson

The F# compiler gives me the following errors for line 20:

This anonymous record does not have enough fields. Add the missing fields [starting].F# Compiler(1)