How should RouterMiddleware be used?

[finlaydotb]

In an application using an older version of oak, the middleware get's plugged into the handler as follows:

router.get("/path", isAdmin, getHandler)

Where isAdmin is something like:

export const isAdmin = async (ctx: RouterContext<"/path">, next: Function) => {
    await next();
}

This no longer compiles. On further investigation I discovered it seems the middleware should now be typed as RouterMiddleware

But when I updated isAdmin to use RouterMiddleware like this:

export const isAdmin = async (ctx: RouterMiddleware<"/path">, next: Function) => {
    await next();
}

It now fails to compile with the following error:

Argument of type '(ctx: RouterMiddleware</path>, next: Function) => Promise<void>' is not assignable to parameter of type 'RouterMiddleware<"/path", Record<string | number, string | undefined>, Record<string, any>>'.

Any ideas on how to fix this?