Cannot iterate over array of router middleware

[callthecapital]

I have a method (bottom of the post) that dynamically generates my api routes but since upgrading Deno's Oak framework to the latest version, the method has stopped working. I've been getting the following error:

error: Uncaught TypeError: undefined is not iterable (cannot read property Symbol(Symbol.iterator))
          ...group.middleware as [],
                   ^

First, here are my types:

import { RouterContext, RouterMiddleware } from "oak";

export type RouteGroup = {
  group: {
    prefix: string;
    middleware: RouterMiddleware<string>[];
  };
  routes: Route[];
};

type Route = {
  method:
    | "all"
    | "delete"
    | "get"
    | "head"
    | "options"
    | "patch"
    | "post"
    | "put";
  path: string;
  middleware: RouterMiddleware<string>[];
  handler: (ctx: RouterContext<string>) => Promise<void>;
};

I had to update my types (above) for the latest version of oak due to RouterMiddleware now requiring that a generic type be passed:

//before
RouterMiddleware[]

// after
RouterMiddleware<string>[]

Here is the method that is throwing the error from above:

private generateRoutes(routeGroups: RouteGroup[]): void {
    routeGroups.forEach(({ group, routes }) => {
      routes.forEach(({ method, path, middleware, handler }) => {
        (this.router[method] as OakRouter["all"])(
          group.prefix + path,
          ...group.middleware as [],      // <-- error on this line
          ...middleware as [],
          handler,
        );
      });
    });
  }

I cannot figure out why my middleware property is now being treated as undefined. How do I turn it back into an iterable?