diff --git a/js/testing/util.ts b/js/testing/util.ts
index 1e245fb712ee7038b95b09c31d847a2cd08932dc..096b184b8bb595ae6f12836ef74433ae1042c7e4 100644
--- a/js/testing/util.ts
+++ b/js/testing/util.ts
@@ -13,19 +13,29 @@
    limitations under the License.
  */
 
-// TODO(ry) Use unknown here for parameters types.
-// tslint:disable-next-line:no-any
-export function assertEqual(actual: any, expected: any, msg?: string) {
-  if (!msg) {
-    msg = `actual: ${actual} expected: ${expected}`;
-  }
+export function assertEqual(actual: unknown, expected: unknown, msg?: string) {
   if (!equal(actual, expected)) {
+    let actualString: string;
+    let expectedString: string;
+    try {
+      actualString = String(actual);
+    } catch (e) {
+      actualString = "[Cannot display]";
+    }
+    try {
+      expectedString = String(expected);
+    } catch (e) {
+      expectedString = "[Cannot display]";
+    }
     console.error(
       "assertEqual failed. actual =",
-      actual,
+      actualString,
       "expected =",
-      expected
+      expectedString
     );
+    if (!msg) {
+      msg = `actual: ${actualString} expected: ${expectedString}`;
+    }
     throw new Error(msg);
   }
 }
diff --git a/js/testing/util_test.ts b/js/testing/util_test.ts
index 9ac3dfd71b3d3e689ba818f747f1f526ac0863ab..25f0b2c45975d5fc87fbfd302c494590bdffc871 100644
--- a/js/testing/util_test.ts
+++ b/js/testing/util_test.ts
@@ -17,7 +17,7 @@ import { test } from "./testing.ts";
 import { assert } from "./util.ts";
 import * as util from "./util.ts";
 
-test(async function util_equal() {
+test(function util_equal() {
   assert(util.equal("world", "world"));
   assert(!util.equal("hello", "world"));
   assert(util.equal(5, 5));
@@ -38,3 +38,35 @@ test(async function util_equal() {
     )
   );
 });
+
+test(function util_assertEqual() {
+  const a = Object.create(null);
+  a.b = "foo";
+  util.assertEqual(a, a);
+});
+
+test(function util_assertEqualActualUncoercable() {
+  let didThrow = false;
+  const a = Object.create(null);
+  try {
+    util.assertEqual(a, "bar");
+  } catch (e) {
+    didThrow = true;
+    console.log(e.message);
+    assert(e.message === "actual: [Cannot display] expected: bar");
+  }
+  assert(didThrow);
+});
+
+test(function util_assertEqualExpectedUncoercable() {
+  let didThrow = false;
+  const a = Object.create(null);
+  try {
+    util.assertEqual("bar", a);
+  } catch (e) {
+    didThrow = true;
+    console.log(e.message);
+    assert(e.message === "actual: bar expected: [Cannot display]");
+  }
+  assert(didThrow);
+});