diff --git a/src/ServiceContainer.ts b/src/ServiceContainer.ts
index bbeb88e..e90353d 100644
--- a/src/ServiceContainer.ts
+++ b/src/ServiceContainer.ts
@@ -24,7 +24,7 @@ export default {
     }
 
     if (!service.instance) {
-      service.instance = service.factory();
+      service.instance = service.factory?.();
       service.factory = undefined;
     }
 
@@ -63,6 +63,6 @@ function getFactory(factoryOrClassReference: Factory | Function): Factory {
 
 type Factory = () => any;
 type Service = {
-  factory: () => any;
+  factory?: () => any;
   instance?: any;
 }
diff --git a/tsconfig.json b/tsconfig.json
index 1d09eec..3aec876 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -6,7 +6,10 @@
     "declaration": true,
     "esModuleInterop": true,
     "moduleResolution": "node",
-    "module": "commonjs"
+    "module": "commonjs",
+    "strict": true,
+    "noImplicitAny": true,
+    "strictPropertyInitialization": true
   },
   "include": [
     "src/**/*"