4 files changed, 14 insertions, 9 deletions
diff --git a/std/fs/copy.ts b/std/fs/copy.ts
index 62e6f59e5..86fac78df 100644
--- a/std/fs/copy.ts
+++ b/std/fs/copy.ts
@@ -85,6 +85,8 @@ async function copyFile(
   await Deno.copyFile(src, dest);
   if (options.preserveTimestamps) {
     const statInfo = await Deno.stat(src);
+    assert(statInfo.accessed != null, `statInfo.accessed is unavailable`);
+    assert(statInfo.modified != null, `statInfo.modified is unavailable`);
     await Deno.utime(dest, statInfo.accessed, statInfo.modified);
   }
 }
diff --git a/std/fs/empty_dir_test.ts b/std/fs/empty_dir_test.ts
index b27ebd033..928ba80e6 100644
--- a/std/fs/empty_dir_test.ts
+++ b/std/fs/empty_dir_test.ts
@@ -1,5 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 import {
+  assert,
   assertEquals,
   assertStrContains,
   assertThrows,
@@ -225,6 +226,8 @@ Deno.test(async function emptyDirPermission(): Promise<void> {
         args: args
       });
 
+      assert(stdout);
+
       const output = await Deno.readAll(stdout);
 
       assertStrContains(new TextDecoder().decode(output), s.output);
diff --git a/std/fs/exists_test.ts b/std/fs/exists_test.ts
index 06b908b9d..204799447 100644
--- a/std/fs/exists_test.ts
+++ b/std/fs/exists_test.ts
@@ -131,7 +131,7 @@ Deno.test(async function existsPermission(): Promise<void> {
       args: args
     });
 
-    const output = await Deno.readAll(stdout);
+    const output = await Deno.readAll(stdout!);
 
     assertStrContains(new TextDecoder().decode(output), s.output);
   }
diff --git a/std/fs/walk.ts b/std/fs/walk.ts
index 108eebc46..890114525 100644
--- a/std/fs/walk.ts
+++ b/std/fs/walk.ts
@@ -65,9 +65,9 @@ export async function* walk(
     includeFiles = true,
     includeDirs = true,
     followSymlinks = false,
-    exts = null,
-    match = null,
-    skip = null
+    exts = undefined,
+    match = undefined,
+    skip = undefined
   }: WalkOptions = {}
 ): AsyncIterableIterator<WalkInfo> {
   if (maxDepth < 0) {
@@ -76,7 +76,7 @@ export async function* walk(
   if (includeDirs && include(root, exts, match, skip)) {
     yield { filename: root, info: await stat(root) };
   }
-  if (maxDepth < 1 || !include(root, null, null, skip)) {
+  if (maxDepth < 1 || !include(root, undefined, undefined, skip)) {
     return;
   }
   const ls: FileInfo[] = await readDir(root);
@@ -119,9 +119,9 @@ export function* walkSync(
     includeFiles = true,
     includeDirs = true,
     followSymlinks = false,
-    exts = null,
-    match = null,
-    skip = null
+    exts = undefined,
+    match = undefined,
+    skip = undefined
   }: WalkOptions = {}
 ): IterableIterator<WalkInfo> {
   if (maxDepth < 0) {
@@ -130,7 +130,7 @@ export function* walkSync(
   if (includeDirs && include(root, exts, match, skip)) {
     yield { filename: root, info: statSync(root) };
   }
-  if (maxDepth < 1 || !include(root, null, null, skip)) {
+  if (maxDepth < 1 || !include(root, undefined, undefined, skip)) {
     return;
   }
   const ls: FileInfo[] = readDirSync(root);