diff --git a/lesson_15/tdd/tdd_app/src/main/java/com/codedifferently/lesson15/EmployeeManager.java b/lesson_15/tdd/tdd_app/src/main/java/com/codedifferently/lesson15/EmployeeManager.java
index 66337295..4ac9edc4 100644
--- a/lesson_15/tdd/tdd_app/src/main/java/com/codedifferently/lesson15/EmployeeManager.java
+++ b/lesson_15/tdd/tdd_app/src/main/java/com/codedifferently/lesson15/EmployeeManager.java
@@ -4,7 +4,8 @@
 import java.util.Map;
 
 public class EmployeeManager {
-  private Map<Integer, Employee> employees;
+
+  private final Map<Integer, Employee> employees;
 
   public EmployeeManager() {
     employees = new HashMap<>();
@@ -15,17 +16,27 @@ public void addEmployee(Employee employee) {
   }
 
   public Employee getEmployee(int id) {
+    assertEmployeeInCollection(id);
     return employees.get(id);
   }
 
   public void updateEmployee(Employee employee) {
+    assertEmployeeInCollection(employee.getId());
     employees.put(employee.getId(), employee);
   }
 
   public void removeEmployee(int id) {
+    assertEmployeeInCollection(id);
     employees.remove(id);
   }
 
+  private void assertEmployeeInCollection(int id) {
+    if (this.employees.containsKey(id)) {
+      return;
+    }
+    throw new IllegalArgumentException("Employee does not in collection with id " + id);
+  }
+
   public int getEmployeeCount() {
     return employees.size();
   }