diff --git a/lib/import_map.ts b/lib/import_map.ts
index 70e30bd2..1d0cee0f 100644
--- a/lib/import_map.ts
+++ b/lib/import_map.ts
@@ -36,21 +36,21 @@ const isImportMapReferrer = is.ObjectOf({
 
 // This implementation is ridiculously inefficient, but we prefer not to reimplement the whole
 // import_map module. Maybe we should rathre patch rust code of the import_map module.
-async function readFromJson(url: URL): Promise<Maybe<ImportMap>> {
-  const data = await Deno.readTextFile(url.pathname);
+async function readFromJson(specifier: URI<"file">): Promise<Maybe<ImportMap>> {
+  const data = await Deno.readTextFile(URI.relative(specifier));
   if (data.length === 0) return;
   const json = parseJsonc(data);
   if (isImportMapReferrer(json)) {
     // The json seems to be deno.json or deno.jsonc referencing an import map.
-    return readFromJson(new URL(json.importMap, url));
+    return readFromJson(URI.from(new URL(json.importMap, specifier).href));
   }
   if (!isImportMapJson(json)) {
     // The json does not include an import map.
     return undefined;
   }
-  const inner = await parseFromJson(url, json);
+  const inner = await parseFromJson(specifier, json);
   return {
-    specifier: URI.from(url.href),
+    specifier,
     resolve(specifier, referrer) {
       const resolved = inner.resolve(specifier, referrer);
       if (resolved === specifier) {
diff --git a/lib/import_map_test.ts b/lib/import_map_test.ts
index 46dbd4ab..65fae968 100644
--- a/lib/import_map_test.ts
+++ b/lib/import_map_test.ts
@@ -11,21 +11,25 @@ describe("readFromJson()", () => {
     // cleanup.defer(async () => {
     //   await Deno.remove(f);
     // });
-    const importMap = await ImportMap.readFromJson(new URL(f, import.meta.url));
+    const importMap = await ImportMap.readFromJson(URI.from(f));
     assertEquals(importMap, undefined);
     await Deno.remove(f);
   });
   it("test/fixtures/import-map/deno.json", async () => {
     const importMap = await ImportMap.readFromJson(
-      new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      URI.from(
+        new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      ),
     );
     assertExists(importMap);
   });
   it("test/fixtures/import-map-referred/import_map.json", async () => {
     const importMap = await ImportMap.readFromJson(
-      new URL(
-        "../test/fixtures/import-map-referred/deno.json",
-        import.meta.url,
+      URI.from(
+        new URL(
+          "../test/fixtures/import-map-referred/deno.json",
+          import.meta.url,
+        ),
       ),
     );
     assertExists(importMap);
@@ -39,10 +43,12 @@ describe("readFromJson()", () => {
 describe("resolve()", () => {
   it("resolve specifiers in import maps", async () => {
     const importMap = await ImportMap.readFromJson(
-      new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      URI.from(
+        new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      ),
     );
     assertExists(importMap);
-    const referrer = URI.from("test/fixtures/import-map/mod.ts");
+    const referrer = URI.from("./test/fixtures/import-map/mod.ts");
     assertEquals(
       importMap.resolve("std/version.ts", referrer),
       {
@@ -70,19 +76,21 @@ describe("resolve()", () => {
     assertEquals(
       importMap.resolve("/lib.ts", referrer),
       {
-        specifier: URI.from("test/fixtures/import-map/lib.ts"),
+        specifier: URI.from("./test/fixtures/import-map/lib.ts"),
       },
     );
   });
   it("do not resolve an url", async () => {
     const importMap = await ImportMap.readFromJson(
-      new URL(
-        "../test/fixtures/import-map-no-resolve/deno.json",
-        import.meta.url,
+      URI.from(
+        new URL(
+          "../test/fixtures/import-map-no-resolve/deno.json",
+          import.meta.url,
+        ),
       ),
     );
     assertExists(importMap);
-    const referrer = URI.from("test/fixtures/import-map-no-resolve/deps.ts");
+    const referrer = URI.from("./test/fixtures/import-map-no-resolve/deps.ts");
     assertEquals(
       importMap.resolve(
         "https://deno.land/std@0.171.0/testing/asserts.ts",
@@ -93,9 +101,11 @@ describe("resolve()", () => {
   });
   it("resolve specifiers in a referred import map", async () => {
     const importMap = await ImportMap.readFromJson(
-      new URL(
-        "../test/fixtures/import-map-referred/deno.json",
-        import.meta.url,
+      URI.from(
+        new URL(
+          "../test/fixtures/import-map-referred/deno.json",
+          import.meta.url,
+        ),
       ),
     );
     assertExists(importMap);
@@ -115,7 +125,9 @@ describe("resolveSimple()", () => {
   let importMap: ImportMap;
   beforeAll(async () => {
     const maybe = await ImportMap.readFromJson(
-      new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      URI.from(
+        new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      ),
     );
     assertExists(maybe);
     importMap = maybe;
diff --git a/lib/update.ts b/lib/update.ts
index 48ef603c..a70563ce 100644
--- a/lib/update.ts
+++ b/lib/update.ts
@@ -74,7 +74,7 @@ export async function collect(
   await DenoGraph.ensureInit();
 
   const importMap = options.importMap
-    ? await ImportMap.readFromJson(new URL(URI.from(options.importMap)))
+    ? await ImportMap.readFromJson(URI.from(options.importMap))
     : undefined;
 
   const graph = await createGraph(specifiers, {
diff --git a/lib/update_test.ts b/lib/update_test.ts
index 4860e9ee..e1862cfc 100644
--- a/lib/update_test.ts
+++ b/lib/update_test.ts
@@ -56,7 +56,9 @@ describe("_create - with import map", () => {
   let importMap: ImportMap;
   beforeAll(async () => {
     importMap = (await ImportMap.readFromJson(
-      new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      URI.from(
+        new URL("../test/fixtures/import-map/deno.json", import.meta.url),
+      ),
     ))!;
   });
   it("std/version.ts", async () => {
diff --git a/lib/uri.ts b/lib/uri.ts
index 29d79910..5a32140a 100644
--- a/lib/uri.ts
+++ b/lib/uri.ts
@@ -1,4 +1,5 @@
 import { isAbsolute, relative, resolve, toFileUrl } from "./std/path.ts";
+import { assert, is } from "./x/unknownutil.ts";
 import { Brand } from "./types.ts";
 
 export type DefaultProtocol<Scheme extends string> = Scheme extends
@@ -15,14 +16,19 @@ export type RelativePath = Brand<string, "RelativePath">;
 export type AbsolutePath = Brand<string, "AbsolutePath">;
 
 export const URI = {
-  from(path: string): URI<"file"> {
+  /**
+   * Convert a path to a file URL. If the path is relative, it is resolved from the current
+   * working directory.
+   */
+  from(path: string | URL): URI<"file"> {
     let url: URL;
     try {
       url = new URL(path);
     } catch {
-      return toFileUrl(
+      assert(path, is.String);
+      url = toFileUrl(
         isAbsolute(path) ? path : resolve(path),
-      ).href as URI<"file">;
+      );
     }
     if (url.protocol !== "file:") {
       throw new TypeError(`Invalid protocol: ${url.protocol} in ${path}`);
diff --git a/lib/x/unknownutil.ts b/lib/x/unknownutil.ts
index 894bcaff..dfddb9a6 100644
--- a/lib/x/unknownutil.ts
+++ b/lib/x/unknownutil.ts
@@ -1 +1 @@
-export { is } from "https://deno.land/x/unknownutil@v3.10.0/mod.ts";
+export { assert, is } from "https://deno.land/x/unknownutil@v3.10.0/mod.ts";