diff --git a/src/main/java/com/leungcheng/spring_simple_backend/controller/AuthController.java b/src/main/java/com/leungcheng/spring_simple_backend/controller/AuthController.java
index 972ba9e..6f671f7 100644
--- a/src/main/java/com/leungcheng/spring_simple_backend/controller/AuthController.java
+++ b/src/main/java/com/leungcheng/spring_simple_backend/controller/AuthController.java
@@ -23,7 +23,7 @@ public class AuthController {
   @PostMapping("/signup")
   @ResponseStatus(HttpStatus.CREATED)
   public void signup(@Valid @RequestBody AuthController.UserCredentials userCredentials) {
-    if (this.userRepository.findByUsername(userCredentials.username()) != null) {
+    if (this.userRepository.findByUsername(userCredentials.username()).isPresent()) {
       throw new UsernameAlreadyExistsException(userCredentials.username());
     }
 
diff --git a/src/main/java/com/leungcheng/spring_simple_backend/domain/CustomUserDetailService.java b/src/main/java/com/leungcheng/spring_simple_backend/domain/CustomUserDetailService.java
index 0b4b954..7a7f571 100644
--- a/src/main/java/com/leungcheng/spring_simple_backend/domain/CustomUserDetailService.java
+++ b/src/main/java/com/leungcheng/spring_simple_backend/domain/CustomUserDetailService.java
@@ -10,6 +10,8 @@ public class CustomUserDetailService implements UserDetailsService {
 
   @Override
   public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
-    return repository.findByUsername(username); // TODO: Handle case that user is not found
+    return repository
+        .findByUsername(username)
+        .orElseThrow(() -> new UsernameNotFoundException(username + " not found"));
   }
 }
diff --git a/src/main/java/com/leungcheng/spring_simple_backend/domain/UserRepository.java b/src/main/java/com/leungcheng/spring_simple_backend/domain/UserRepository.java
index a355e8b..f695836 100644
--- a/src/main/java/com/leungcheng/spring_simple_backend/domain/UserRepository.java
+++ b/src/main/java/com/leungcheng/spring_simple_backend/domain/UserRepository.java
@@ -1,7 +1,8 @@
 package com.leungcheng.spring_simple_backend.domain;
 
+import java.util.Optional;
 import org.springframework.data.repository.CrudRepository;
 
 public interface UserRepository extends CrudRepository<User, String> {
-  User findByUsername(String username);
+  Optional<User> findByUsername(String username);
 }