Based on the error message and your code, it seems that you are encountering an issue when trying to traverse a lazily-loaded relationship between Student and Module entities without properly initializing the relationship. The error occurs when the line student.addModule(module); is executed because the lazy relationship between Student and Module has not been instantiated yet. To address this issue, I would recommend making sure that the Student and Module classes are properly annotated with @ManyToMany and @JoinTable annotations. Here's an example of how they should be set up: ``` @Entity public class Student { @Id private Long id; @ManyToMany(fetch = FetchType.LAZY) @JoinTable( name = "student_module", joinColumns = @JoinColumn(name = "student_id"), inverseJoinColumns = @JoinColumn(name = "module_id") ) private List<Module> modules = new ArrayList<>(); // Getters, setters, and other methods } @Entity public class Module { @Id private Long id; @ManyToMany(mappedBy = "modules", fetch = FetchType.LAZY) private List<Student> students = new ArrayList<>(); // Getters, setters, and other methods } ``` Once you have ensured that your entities are set up correctly, there are a couple of ways to resolve the issue: 1. Eagerly fetch the relationship: Change the FetchType to EAGER in both Student and Module entities. This will load the relationship whenever a Student or Module is loaded from the database, so you won't encounter the null session issue. However, this approach may not be optimal from a performance standpoint, as it will load all related entities even if they are not needed.