diff --git a/src/lib/extern/kwin.ts b/src/lib/extern/kwin.ts
index 53643b2..fd4eb0e 100644
--- a/src/lib/extern/kwin.ts
+++ b/src/lib/extern/kwin.ts
@@ -1,8 +1,12 @@
 type KWin = {
+    __brand: "KWin";
+
     readConfig(key: string, defaultValue: any): any;
 };
 
 type Workspace = {
+    __brand: "Workspace";
+
     readonly activities: string[];
     readonly desktops: KwinDesktop[];
     readonly currentDesktop: KwinDesktop;
@@ -44,10 +48,12 @@ const enum MaximizedMode {
     Maximized,
 }
 
-type Tile = unknown;
-type Output = unknown;
+type Tile = { __brand: "Tile" };
+type Output = { __brand: "Output" };
 
 type KwinClient = {
+    __brand: "KwinClient";
+
     readonly shadeable: boolean;
     readonly caption: string;
     readonly minSize: Readonly<QmlSize>;
@@ -77,7 +83,7 @@ type KwinClient = {
     minimized: boolean;
     frameGeometry: QmlRect;
     desktops: KwinDesktop[]; // empty array means all desktops
-    tile: Tile;
+    tile: Tile|null;
     opacity: number;
 
     readonly fullScreenChanged: QSignal<[]>;
@@ -95,10 +101,12 @@ type KwinClient = {
 };
 
 type KwinDesktop = {
+    __brand: "KwinDesktop";
+
     readonly id: string;
 };
 
-type ShortcutHandler = {
+type ShortcutHandler = QmlObject & {
     readonly activated: QSignal<[]>;
     destroy(): void;
 };
diff --git a/src/lib/extern/notification.ts b/src/lib/extern/notification.ts
index 860a4cb..c73dddb 100644
--- a/src/lib/extern/notification.ts
+++ b/src/lib/extern/notification.ts
@@ -1,3 +1,3 @@
-type Notification = {
+type Notification = QmlObject & {
     sendEvent(): void;
 };
diff --git a/src/lib/extern/qt.ts b/src/lib/extern/qt.ts
index 055eb50..39adb56 100644
--- a/src/lib/extern/qt.ts
+++ b/src/lib/extern/qt.ts
@@ -1,21 +1,29 @@
 type Console = {
+    __brand: "Console";
+
     log(...args: any[]): void;
     assert(assertion: boolean, message?: string): void;
 };
 
 type Qt = {
+    __brand: "Qt";
+
     rect(x: number, y: number, width: number, height: number): QmlRect;
     createQmlObject(qml: string, parent: QmlObject): QmlObject;
 };
 
-type QmlObject = unknown;
+type QmlObject = { __brand: "QmlObject" };
 
 type QmlPoint = {
+    __brand: "QmlPoint";
+
     x: number;
     y: number;
 };
 
 type QmlRect = {
+    __brand: "QmlRect";
+
     x: number;
     y: number;
     width: number;
@@ -27,16 +35,20 @@ type QmlRect = {
 };
 
 type QmlSize = {
+    __brand: "QmlSize";
+
     width: number;
     height: number;
 };
 
 type QSignal<T extends unknown[]> = {
+    __brand: "QSignal";
+
     connect(handler: (...args: [...T]) => void): void;
     disconnect(handler: (...args: [...T]) => void): void;
 };
 
-type QmlTimer = {
+type QmlTimer = QmlObject & {
     interval: number;
     readonly triggered: QSignal<[]>;
     restart(): void;
diff --git a/src/tests/utils/mocks/MockKwinClient.ts b/src/tests/utils/mocks/MockKwinClient.ts
index 9004468..58dc809 100644
--- a/src/tests/utils/mocks/MockKwinClient.ts
+++ b/src/tests/utils/mocks/MockKwinClient.ts
@@ -1,4 +1,6 @@
 class MockKwinClient {
+    public readonly __brand = "KwinClient";
+
     private static readonly borderThickness = 10;
 
     public readonly shadeable: boolean = false;
@@ -10,7 +12,7 @@ class MockKwinClient {
     public readonly resizeable: boolean = true;
     public readonly fullScreenable: boolean = true;
     public readonly maximizable: boolean = true;
-    public readonly output: Output = false;
+    public readonly output: Output = { __brand: "Output" };
     public readonly dock: boolean = false;
     public readonly normalWindow: boolean = true;
     public readonly managed: boolean = true;
@@ -24,7 +26,7 @@ class MockKwinClient {
     public shade: boolean = false;
     public minimized: boolean = false;
     public desktops: KwinDesktop[] = [];
-    public tile: Tile = false;
+    public tile: Tile|null = null;
     public opacity: number = 1.0;
 
     public readonly fullScreenChanged = new MockQSignal();
diff --git a/src/tests/utils/mocks/MockQSignal.ts b/src/tests/utils/mocks/MockQSignal.ts
index a1d0f58..9055da7 100644
--- a/src/tests/utils/mocks/MockQSignal.ts
+++ b/src/tests/utils/mocks/MockQSignal.ts
@@ -1,4 +1,6 @@
 class MockQSignal<T extends unknown[]> {
+    public readonly __brand = "QSignal";
+
     private readonly handlers: Set<(...args: [...T]) => void> = new Set();
 
     public connect(handler: (...args: [...T]) => void) {
diff --git a/src/tests/utils/mocks/MockQmlPoint.ts b/src/tests/utils/mocks/MockQmlPoint.ts
index 485f603..4d7652f 100644
--- a/src/tests/utils/mocks/MockQmlPoint.ts
+++ b/src/tests/utils/mocks/MockQmlPoint.ts
@@ -1,4 +1,6 @@
 class MockQmlPoint {
+    public readonly __brand = "QmlPoint";
+
     constructor(
         public x: number,
         public y: number,
diff --git a/src/tests/utils/mocks/MockQmlRect.ts b/src/tests/utils/mocks/MockQmlRect.ts
index 81ab44f..6800fdb 100644
--- a/src/tests/utils/mocks/MockQmlRect.ts
+++ b/src/tests/utils/mocks/MockQmlRect.ts
@@ -1,4 +1,6 @@
 class MockQmlRect {
+    public readonly __brand = "QmlRect";
+
     constructor(
         private _x: number,
         private _y: number,
diff --git a/src/tests/utils/mocks/MockQmlSize.ts b/src/tests/utils/mocks/MockQmlSize.ts
index 5600a39..f52d6b2 100644
--- a/src/tests/utils/mocks/MockQmlSize.ts
+++ b/src/tests/utils/mocks/MockQmlSize.ts
@@ -1,4 +1,6 @@
 class MockQmlSize {
+    public readonly __brand = "QmlSize";
+
     constructor(
         public width: number,
         public height: number,
diff --git a/src/tests/utils/mocks/MockQmlTimer.ts b/src/tests/utils/mocks/MockQmlTimer.ts
index 1b9aad7..4280176 100644
--- a/src/tests/utils/mocks/MockQmlTimer.ts
+++ b/src/tests/utils/mocks/MockQmlTimer.ts
@@ -1,4 +1,6 @@
 class MockQmlTimer {
+    public readonly __brand = "QmlObject";
+
     public interval = 0;
     public readonly triggered = new MockQSignal();
 
diff --git a/src/tests/utils/mocks/MockQt.ts b/src/tests/utils/mocks/MockQt.ts
index 9fa8c8a..5f32a6e 100644
--- a/src/tests/utils/mocks/MockQt.ts
+++ b/src/tests/utils/mocks/MockQt.ts
@@ -1,4 +1,6 @@
 class MockQt {
+    public readonly __brand = "Qt";
+
     private shortcuts = new Map<string, MockShortcutHandler>();
 
     public point(x: number, y: number) {
@@ -9,7 +11,7 @@ class MockQt {
         return new MockQmlRect(x, y, width, height);
     }
 
-    public createQmlObject(qml: string, parent: QmlObject) {
+    public createQmlObject(qml: string, parent: QmlObject): QmlObject {
         if (qml.includes("Timer")) {
             return new MockQmlTimer();
         } else if (qml.includes("ShortcutHandler")) {
@@ -18,7 +20,7 @@ class MockQt {
             this.shortcuts.set(shortcutName, shortcutHandler);
             return shortcutHandler;
         } else {
-            assert(false, "Unexpected qml string: " + qml);
+            throw new Error("Unexpected qml string: " + qml);
         }
     }
 
diff --git a/src/tests/utils/mocks/MockShortcutHandler.ts b/src/tests/utils/mocks/MockShortcutHandler.ts
index fb85b46..6f33ad3 100644
--- a/src/tests/utils/mocks/MockShortcutHandler.ts
+++ b/src/tests/utils/mocks/MockShortcutHandler.ts
@@ -1,4 +1,6 @@
 class MockShortcutHandler {
+    public readonly __brand = "QmlObject";
+
     public readonly activated: MockQSignal<[]> = new MockQSignal();
 
     public destroy() {}
diff --git a/src/tests/utils/mocks/MockWorkspace.ts b/src/tests/utils/mocks/MockWorkspace.ts
index 5338923..1d5e671 100644
--- a/src/tests/utils/mocks/MockWorkspace.ts
+++ b/src/tests/utils/mocks/MockWorkspace.ts
@@ -1,9 +1,14 @@
 class MockWorkspace {
+    public readonly __brand = "Workspace";
+
     public activities = ["test-activity"];
-    public desktops = [{id: "desktop1"}, {id: "desktop2"}];
+    public desktops: KwinDesktop[] = [
+        { __brand: "KwinDesktop", id: "desktop1" },
+        { __brand: "KwinDesktop", id: "desktop2" }
+    ];
     public currentDesktop = this.desktops[0];
     public currentActivity = this.activities[0];
-    public activeScreen = {};
+    public activeScreen: Output = { __brand: "Output" };
     public windows = [];
     public cursorPos = new MockQmlPoint(0, 0);