eszip build fails with: Error: Blocked by null entry for ""https://deno.land/x/""

[deckchairlabs]

Hello!

When I attempt to build an eszip with the following importMap and a very simple module. I receive the error below.

mod.ts

import 'std/fs/mod.ts';
import 'https://deno.land/x/fresh/server.ts'

importMap.json

{
    "imports": {
        "std/": "https://deno.land/std/",
        "https://deno.land/x/": "./vendor/deno.land/x/"
    }
}

error: Error: Blocked by null entry for ""https://deno.land/x/""
      const ret = new Error(getStringFromWasm0(arg0, arg1));
                  ^
    at __wbg_new_651776e932b7e9c7 (file:///workspaces/eszip/lib/eszip_wasm.generated.js:313:19)
    at <anonymous> (wasm://wasm/00a90a36:1:78732)
    at <anonymous> (wasm://wasm/00a90a36:1:1463894)
    at <anonymous> (wasm://wasm/00a90a36:1:1957066)
    at __wbg_adapter_18 (file:///workspaces/eszip/lib/eszip_wasm.generated.js:144:6)
    at real (file:///workspaces/eszip/lib/eszip_wasm.generated.js:128:14)