diff --git a/js/dispatch_json.ts b/js/dispatch_json.ts
index bde2b7cb1d..8ec924f4c3 100644
--- a/js/dispatch_json.ts
+++ b/js/dispatch_json.ts
@@ -15,10 +15,10 @@ interface JsonError {
 interface JsonResponse {
   ok?: Ok;
   err?: JsonError;
-  promiseId?: number; // only present in async mesasges.
+  promiseId?: number; // Only present in async messages.
 }
 
-const promiseTable = new Map<number, util.Resolvable<number>>();
+const promiseTable = new Map<number, util.Resolvable<JsonResponse>>();
 let _nextPromiseId = 1;
 
 function nextPromiseId(): number {
@@ -35,25 +35,22 @@ function encode(args: object): Uint8Array {
   return new TextEncoder().encode(s);
 }
 
-function toDenoError(err: JsonError): DenoError<ErrorKind> {
-  return new DenoError(err.kind, err.message);
+function unwrapResponse(res: JsonResponse): Ok {
+  if (res.err != null) {
+    throw new DenoError(res.err!.kind, res.err!.message);
+  }
+  util.assert(res.ok != null);
+  return res.ok!;
 }
 
-export function asyncMsgFromRust(opId: number, res: Uint8Array): void {
-  const { ok, err, promiseId } = decode(res);
-  const promise = promiseTable.get(promiseId!)!;
-  if (!promise) {
-    throw Error(`Async op ${opId} had bad promiseId`);
-  }
-  promiseTable.delete(promiseId!);
+export function asyncMsgFromRust(opId: number, resUi8: Uint8Array): void {
+  const res = decode(resUi8);
+  util.assert(res.promiseId != null);
 
-  if (err) {
-    promise.reject(toDenoError(err));
-  } else if (ok) {
-    promise.resolve(ok);
-  } else {
-    util.unreachable();
-  }
+  const promise = promiseTable.get(res.promiseId!);
+  util.assert(promise != null);
+  promiseTable.delete(res.promiseId!);
+  promise!.resolve(res);
 }
 
 export function sendSync(
@@ -62,29 +59,28 @@ export function sendSync(
   zeroCopy?: Uint8Array
 ): Ok {
   const argsUi8 = encode(args);
-  const res = core.dispatch(opId, argsUi8, zeroCopy);
-  if (!res) {
-    return;
-  }
-  const { ok, err, promiseId } = decode(res);
-  util.assert(!promiseId);
-  if (err) {
-    throw toDenoError(err);
-  }
-  return ok;
+  const resUi8 = core.dispatch(opId, argsUi8, zeroCopy);
+  util.assert(resUi8 != null);
+
+  const res = decode(resUi8!);
+  util.assert(res.promiseId == null);
+  return unwrapResponse(res);
 }
 
-export function sendAsync(
+export async function sendAsync(
   opId: number,
   args: object = {},
   zeroCopy?: Uint8Array
 ): Promise<Ok> {
   const promiseId = nextPromiseId();
   args = Object.assign(args, { promiseId });
-  const argsUi8 = encode(args);
   const promise = util.createResolvable<Ok>();
   promiseTable.set(promiseId, promise);
-  const r = core.dispatch(opId, argsUi8, zeroCopy);
-  util.assert(!r);
-  return promise;
+
+  const argsUi8 = encode(args);
+  const resUi8 = core.dispatch(opId, argsUi8, zeroCopy);
+  util.assert(resUi8 == null);
+
+  const res = await promise;
+  return unwrapResponse(res);
 }
diff --git a/js/dispatch_json_test.ts b/js/dispatch_json_test.ts
new file mode 100644
index 0000000000..47e5cef2fa
--- /dev/null
+++ b/js/dispatch_json_test.ts
@@ -0,0 +1,19 @@
+import { testPerm, assertMatch, unreachable } from "./test_util.ts";
+
+const openErrorStackPattern = new RegExp(
+  `^.*
+    at unwrapResponse \\(js\\/dispatch_json\\.ts:.*\\)
+    at sendAsync.* \\(js\\/dispatch_json\\.ts:.*\\)
+    at async Object\\.open \\(js\\/files\\.ts:.*\\).*$`,
+  "ms"
+);
+
+testPerm({ read: true }, async function sendAsyncStackTrace(): Promise<void> {
+  await Deno.open("nonexistent.txt")
+    .then(unreachable)
+    .catch(
+      (error): void => {
+        assertMatch(error.stack, openErrorStackPattern);
+      }
+    );
+});
diff --git a/js/test_util.ts b/js/test_util.ts
index 454f26ff21..bcbacf2812 100644
--- a/js/test_util.ts
+++ b/js/test_util.ts
@@ -15,9 +15,11 @@ import {
 export {
   assert,
   assertEquals,
+  assertMatch,
   assertNotEquals,
   assertStrictEq,
-  assertStrContains
+  assertStrContains,
+  unreachable
 } from "./deps/https/deno.land/std/testing/asserts.ts";
 
 interface TestPermissions {
diff --git a/js/unit_tests.ts b/js/unit_tests.ts
index b55c1954aa..711a092fdb 100644
--- a/js/unit_tests.ts
+++ b/js/unit_tests.ts
@@ -13,6 +13,7 @@ import "./console_test.ts";
 import "./copy_file_test.ts";
 import "./custom_event_test.ts";
 import "./dir_test.ts";
+import "./dispatch_json_test.ts";
 import "./error_stack_test.ts";
 import "./event_test.ts";
 import "./event_target_test.ts";
diff --git a/tests/error_004_missing_module.ts.out b/tests/error_004_missing_module.ts.out
index 261d0a97bc..a0ce342e94 100644
--- a/tests/error_004_missing_module.ts.out
+++ b/tests/error_004_missing_module.ts.out
@@ -1,7 +1,7 @@
 [WILDCARD]error: Uncaught NotFound: Cannot resolve module "[WILDCARD]/bad-module.ts"
 [WILDCARD] js/dispatch_json.ts:[WILDCARD]
     at DenoError (js/errors.ts:[WILDCARD])
-    at toDenoError (js/dispatch_json.ts:[WILDCARD])
+    at unwrapResponse (js/dispatch_json.ts:[WILDCARD])
     at sendSync[WILDCARD] (js/dispatch_json.ts:[WILDCARD])
     at fetchSourceFile (js/compiler.ts:[WILDCARD])
     at _resolveModule (js/compiler.ts:[WILDCARD])
diff --git a/tests/error_005_missing_dynamic_import.ts.out b/tests/error_005_missing_dynamic_import.ts.out
index 6574705313..a580368bae 100644
--- a/tests/error_005_missing_dynamic_import.ts.out
+++ b/tests/error_005_missing_dynamic_import.ts.out
@@ -1,7 +1,7 @@
 [WILDCARD]error: Uncaught NotFound: Cannot resolve module "[WILDCARD]/bad-module.ts"
 [WILDCARD] js/dispatch_json.ts:[WILDCARD]
     at DenoError (js/errors.ts:[WILDCARD])
-    at toDenoError (js/dispatch_json.ts:[WILDCARD])
+    at unwrapResponse (js/dispatch_json.ts:[WILDCARD])
     at sendSync[WILDCARD] (js/dispatch_json.ts:[WILDCARD])
     at fetchSourceFile (js/compiler.ts:[WILDCARD])
     at _resolveModule (js/compiler.ts:[WILDCARD])
diff --git a/tests/error_006_import_ext_failure.ts.out b/tests/error_006_import_ext_failure.ts.out
index 6ccee7e13b..442ef9c53d 100644
--- a/tests/error_006_import_ext_failure.ts.out
+++ b/tests/error_006_import_ext_failure.ts.out
@@ -1,7 +1,7 @@
 [WILDCARD]error: Uncaught NotFound: Cannot resolve module "[WILDCARD]/non-existent"
 [WILDCARD] js/dispatch_json.ts:[WILDCARD]
     at DenoError (js/errors.ts:[WILDCARD])
-    at toDenoError (js/dispatch_json.ts:[WILDCARD])
+    at unwrapResponse (js/dispatch_json.ts:[WILDCARD])
     at sendSync[WILDCARD] (js/dispatch_json.ts:[WILDCARD])
     at fetchSourceFile (js/compiler.ts:[WILDCARD])
     at _resolveModule (js/compiler.ts:[WILDCARD])
diff --git a/tests/error_011_bad_module_specifier.ts.out b/tests/error_011_bad_module_specifier.ts.out
index 3b7baa2414..518ad08b3f 100644
--- a/tests/error_011_bad_module_specifier.ts.out
+++ b/tests/error_011_bad_module_specifier.ts.out
@@ -1,7 +1,7 @@
 [WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "bad-module.ts" not prefixed with / or ./ or ../
 [WILDCARD] js/dispatch_json.ts:[WILDCARD]
     at DenoError (js/errors.ts:[WILDCARD])
-    at toDenoError (js/dispatch_json.ts:[WILDCARD])
+    at unwrapResponse (js/dispatch_json.ts:[WILDCARD])
     at sendSync[WILDCARD] (js/dispatch_json.ts:[WILDCARD])
     at fetchSourceFile (js/compiler.ts:[WILDCARD])
     at _resolveModule (js/compiler.ts:[WILDCARD])
diff --git a/tests/error_012_bad_dynamic_import_specifier.ts.out b/tests/error_012_bad_dynamic_import_specifier.ts.out
index 3b7baa2414..518ad08b3f 100644
--- a/tests/error_012_bad_dynamic_import_specifier.ts.out
+++ b/tests/error_012_bad_dynamic_import_specifier.ts.out
@@ -1,7 +1,7 @@
 [WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "bad-module.ts" not prefixed with / or ./ or ../
 [WILDCARD] js/dispatch_json.ts:[WILDCARD]
     at DenoError (js/errors.ts:[WILDCARD])
-    at toDenoError (js/dispatch_json.ts:[WILDCARD])
+    at unwrapResponse (js/dispatch_json.ts:[WILDCARD])
     at sendSync[WILDCARD] (js/dispatch_json.ts:[WILDCARD])
     at fetchSourceFile (js/compiler.ts:[WILDCARD])
     at _resolveModule (js/compiler.ts:[WILDCARD])