diff --git a/WD7UVN_HFT_2023241.Logic/LogicServices.cs b/WD7UVN_HFT_2023241.Logic/LogicServices.cs
index dfb406c..6a4bb26 100644
--- a/WD7UVN_HFT_2023241.Logic/LogicServices.cs
+++ b/WD7UVN_HFT_2023241.Logic/LogicServices.cs
@@ -26,7 +26,7 @@ namespace WD7UVN_HFT_2023241.Logic
             this.CRUDOperations = CRUDOperations;
         }
 
-		public IQueryable<Employee> WhoMaintainsService(int serviceId)
+		public IQueryable<Employee>? WhoMaintainsService(int serviceId)
 		{
 			IQueryable<Employee> query =
 				from employee in CRUDOperations.ReadAllEmployees()
@@ -34,10 +34,15 @@ namespace WD7UVN_HFT_2023241.Logic
 				where service.ID == serviceId
 				select employee;
 
+			if (query.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+			
 			return query;
 		}
 
-		public Employee WhoIsResponsibleForService(int serviceId)
+		public Employee? WhoIsResponsibleForService(int serviceId)
 		{
 			var query =
 				from service in CRUDOperations.ReadAllServices()
@@ -46,28 +51,54 @@ namespace WD7UVN_HFT_2023241.Logic
 				where service.ID == serviceId
 				select employee;
 
+			if (query.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+            
 			return query.FirstOrDefault();
 		}
 
-public IQueryable<Employee> WhoWorksInMaintainerTeam(int maintainerTeamId)
+        public IQueryable<Employee>? WhoWorksInMaintainerTeam(int maintainerTeamId)
         {
-            return CRUDOperations
+            var res = CRUDOperations
             .ReadAllEmployees()
             .Where(e => e.MAINTAINER_ID == maintainerTeamId);
+
+			if (res.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+
+            return res;
         }
 
-        public IQueryable<Employee> GetSubordinates(int managerId)
+        public IQueryable<Employee>? GetSubordinates(int managerId)
         {
-            return CRUDOperations
+            var res = CRUDOperations
             .ReadAllEmployees()
             .Where(e => e.MANAGER_ID == managerId);
+
+			if (res.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+
+            return res;
         }
 
-        public IQueryable<Customer> WhoUsesService(int serviceId)
+        public IQueryable<Customer>? WhoUsesService(int serviceId)
         {
-            return CRUDOperations
+            var res = CRUDOperations
             .ReadAllCustomers()
             .Where(c => c.SERVICE_ID == serviceId);
+
+            if (res.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+
+            return res;
         }
     }
 }
diff --git a/WD7UVN_HFT_2023241.Repository/CRUD.cs b/WD7UVN_HFT_2023241.Repository/CRUD.cs
index 2dcccb7..5638f8c 100644
--- a/WD7UVN_HFT_2023241.Repository/CRUD.cs
+++ b/WD7UVN_HFT_2023241.Repository/CRUD.cs
@@ -52,12 +52,12 @@ namespace WD7UVN_HFT_2023241.Repository
             Database.Context.SaveChanges();
         }
 
-        public Employee ReadEmployee(int employeeId)
+        public Employee? ReadEmployee(int employeeId)
         {
             return Database.Context.Employees.Find(employeeId);
         }
 
-        public IQueryable<Employee> ReadAllEmployees()
+        public IQueryable<Employee>? ReadAllEmployees()
         {
             return Database.Context.Employees.AsQueryable();
         }
@@ -88,12 +88,12 @@ namespace WD7UVN_HFT_2023241.Repository
             Database.Context.SaveChanges();
         }
 
-        public Service ReadService(int serviceId)
+        public Service? ReadService(int serviceId)
         {
             return Database.Context.Services.Find(serviceId);
         }
 
-        public IQueryable<Service> ReadAllServices()
+        public IQueryable<Service>? ReadAllServices()
         {
             return Database.Context.Services.AsQueryable();
         }
@@ -124,12 +124,12 @@ namespace WD7UVN_HFT_2023241.Repository
             Database.Context.SaveChanges();
         }
 
-        public MaintainerTeam ReadMaintainerTeam(int maintainerTeamId)
+        public MaintainerTeam? ReadMaintainerTeam(int maintainerTeamId)
         {
             return Database.Context.Maintainers.Find(maintainerTeamId);
         }
 
-        public IQueryable<MaintainerTeam> ReadAllMaintainerTeams()
+        public IQueryable<MaintainerTeam>? ReadAllMaintainerTeams()
         {
             return Database.Context.Maintainers.AsQueryable();
         }
@@ -160,12 +160,12 @@ namespace WD7UVN_HFT_2023241.Repository
             Database.Context.SaveChanges();
         }
 
-        public Customer ReadCustomer(int customerId)
+        public Customer? ReadCustomer(int customerId)
         {
             return Database.Context.Customers.Find(customerId);
         }
 
-        public IQueryable<Customer> ReadAllCustomers()
+        public IQueryable<Customer>? ReadAllCustomers()
         {
             return Database.Context.Customers.AsQueryable();
         }