3 files changed, 7 insertions, 1 deletions
diff --git a/cli/tests/unit/opcall_test.ts b/cli/tests/unit/opcall_test.ts
index 7e3685320..e38f481d9 100644
--- a/cli/tests/unit/opcall_test.ts
+++ b/cli/tests/unit/opcall_test.ts
@@ -12,7 +12,9 @@ unitTest(async function sendAsyncStackTrace() {
     await Deno.read(rid, buf);
     unreachable();
   } catch (error) {
-    const s = error.stack.toString();
+    assert(error instanceof Error);
+    const s = error.stack?.toString();
+    assert(s);
     console.log(s);
     assertStringIncludes(s, "opcall_test.ts");
     assertStringIncludes(s, "read");
diff --git a/cli/tests/unit/text_encoding_test.ts b/cli/tests/unit/text_encoding_test.ts
index a65a4176b..d83d84584 100644
--- a/cli/tests/unit/text_encoding_test.ts
+++ b/cli/tests/unit/text_encoding_test.ts
@@ -98,6 +98,7 @@ unitTest(function textDecoderErrorEncoding() {
     new TextDecoder("Foo");
   } catch (e) {
     didThrow = true;
+    assert(e instanceof Error);
     assertEquals(e.message, "The encoding label provided ('Foo') is invalid.");
   }
   assert(didThrow);
diff --git a/cli/tests/unit/write_file_test.ts b/cli/tests/unit/write_file_test.ts
index 0f99e2749..9ad8da578 100644
--- a/cli/tests/unit/write_file_test.ts
+++ b/cli/tests/unit/write_file_test.ts
@@ -1,5 +1,6 @@
 // Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
 import {
+  assert,
   assertEquals,
   assertRejects,
   assertThrows,
@@ -251,6 +252,7 @@ unitTest(
     try {
       await Deno.writeFile(filename, data, { signal: ac.signal });
     } catch (e) {
+      assert(e instanceof Error);
       assertEquals(e.name, "AbortError");
     }
     const stat = Deno.statSync(filename);
@@ -269,6 +271,7 @@ unitTest(
     try {
       await Deno.writeFile(filename, data, { signal: ac.signal });
     } catch (e) {
+      assert(e instanceof Error);
       assertEquals(e.name, "AbortError");
     }
     const stat = Deno.statSync(filename);