diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index 33ac650937a..1961c4ea5fc 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -431,6 +431,7 @@
 		"./vs/platform/update/node/update.config.contribution.ts",
 		"./vs/platform/url/common/url.ts",
 		"./vs/platform/url/common/urlService.ts",
+		"./vs/platform/windows/common/windows.ts",
 		"./vs/platform/workbench/common/contextkeys.ts",
 		"./vs/platform/workspace/common/workspace.ts",
 		"./vs/platform/workspace/test/common/testWorkspace.ts",
diff --git a/src/vs/platform/storage/common/storage.ts b/src/vs/platform/storage/common/storage.ts
index fb6e3fd3a0c..913e2f0610a 100644
--- a/src/vs/platform/storage/common/storage.ts
+++ b/src/vs/platform/storage/common/storage.ts
@@ -30,7 +30,7 @@ export interface IStorageService {
 	 * The scope argument allows to define the scope of the storage
 	 * operation to either the current workspace only or all workspaces.
 	 */
-	get(key: string, scope: StorageScope, fallbackValue?: string): string | undefined;
+	get<R extends string | undefined>(key: string, scope: StorageScope, fallbackValue?: R): R;
 
 	/**
 	 * Retrieve an element stored with the given key from storage. Use
@@ -40,7 +40,7 @@ export interface IStorageService {
 	 * The scope argument allows to define the scope of the storage
 	 * operation to either the current workspace only or all workspaces.
 	 */
-	getBoolean(key: string, scope: StorageScope, fallbackValue?: boolean): boolean | undefined;
+	getBoolean<R extends boolean | undefined>(key: string, scope: StorageScope, fallbackValue?: R): R;
 
 	/**
 	 * Retrieve an element stored with the given key from storage. Use
@@ -50,7 +50,7 @@ export interface IStorageService {
 	 * The scope argument allows to define the scope of the storage
 	 * operation to either the current workspace only or all workspaces.
 	 */
-	getInteger(key: string, scope: StorageScope, fallbackValue?: number): number | undefined;
+	getInteger<R extends number | undefined>(key: string, scope: StorageScope, fallbackValue?: R): R;
 
 	/**
 	 * Store a string value under the given key to storage. The value will
@@ -94,16 +94,16 @@ export const NullStorageService: IStorageService = {
 	onDidChangeStorage: Event.None,
 	onWillSaveState: Event.None,
 
-	get(key: string, scope: StorageScope, fallbackValue?: string): string | undefined {
-		return fallbackValue;
+	get<R extends string | undefined>(key: string, scope: StorageScope, fallbackValue?: R): R {
+		return fallbackValue as R;
 	},
 
-	getBoolean(key: string, scope: StorageScope, fallbackValue?: boolean): boolean | undefined {
-		return fallbackValue;
+	getBoolean<R extends boolean | undefined>(key: string, scope: StorageScope, fallbackValue?: R): R {
+		return fallbackValue as R;
 	},
 
-	getInteger(key: string, scope: StorageScope, fallbackValue?: number): number | undefined {
-		return fallbackValue;
+	getInteger<R extends number | undefined>(key: string, scope: StorageScope, fallbackValue?: R): R {
+		return fallbackValue as R;
 	},
 
 	store(key: string, value: any, scope: StorageScope): Promise<void> {
diff --git a/src/vs/platform/windows/common/windows.ts b/src/vs/platform/windows/common/windows.ts
index facb0567866..7b0472e8d94 100644
--- a/src/vs/platform/windows/common/windows.ts
+++ b/src/vs/platform/windows/common/windows.ts
@@ -379,7 +379,7 @@ export class ActiveWindowManager implements IDisposable {
 			.then(id => (typeof this._activeWindowId === 'undefined') && this.setActiveWindow(id));
 	}
 
-	private setActiveWindow(windowId: number) {
+	private setActiveWindow(windowId: number | undefined) {
 		if (this.firstActiveWindowIdPromise) {
 			this.firstActiveWindowIdPromise = null;
 		}