context assert error "Assertions require every name in the call target to be declared with an explicit type annotation"

[nameOfUser0236]

oak's context's assert function generates the following error:
Assertions require every name in the call target to be declared with an explicit type annotation
as shown by this example:

import * as oak from "https://deno.land/x/oak/mod.ts";

const app = new oak.Application();

app.use((ctx) => {
    ctx.assert(true);
});

await app.listen({ port: 8000 });

I suspect that it is because the condition is any.
Some help would be appreciated as my current workaround is very painful.