diff --git a/fsprovider-sample/src/fileSystemProvider.ts b/fsprovider-sample/src/fileSystemProvider.ts
index c6699d1a..68844887 100644
--- a/fsprovider-sample/src/fileSystemProvider.ts
+++ b/fsprovider-sample/src/fileSystemProvider.ts
@@ -152,14 +152,16 @@ export class MemFS implements vscode.FileSystemProvider {
 
     // --- lookup
 
-    private _lookup(uri: vscode.Uri, silent: boolean): Entry {
+    private _lookup(uri: vscode.Uri, silent: false): Entry;
+    private _lookup(uri: vscode.Uri, silent: boolean): Entry | undefined;
+    private _lookup(uri: vscode.Uri, silent: boolean): Entry | undefined {
         let parts = uri.path.split('/');
         let entry: Entry = this.root;
         for (const part of parts) {
             if (!part) {
                 continue;
             }
-            let child: Entry;
+            let child: Entry | undefined;
             if (entry instanceof Directory) {
                 child = entry.entries.get(part);
             }
diff --git a/fsprovider-sample/tsconfig.json b/fsprovider-sample/tsconfig.json
index 11282c9a..79b70855 100644
--- a/fsprovider-sample/tsconfig.json
+++ b/fsprovider-sample/tsconfig.json
@@ -1,5 +1,6 @@
 {
     "compilerOptions": {
+        "strictNullChecks": true,
         "module": "commonjs",
         "target": "es6",
         "outDir": "out",
@@ -13,4 +14,4 @@
         "node_modules",
         ".vscode-test"
     ]
-}
\ No newline at end of file
+}