Cannot save an object with GrantedAuthority field (no Creators, like default construct, exist)

I have been trying to enable a form-based user registration in my project.

Just registering a user with no roles works fine, however adding some roles gives me an error:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of org.springframework.security.core.GrantedAuthority (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information at [Source: (PushbackInputStream); line: 9, column: 21] (through reference chain: com.warehousing.model.User["authorities"]->java.util.HashSet[0])

I suspect that the problem is coming from the fact that Spring has no idea how to convert from my authority representation to something that extends GrantedAuthority. That being said, I tried switching from GrantedAuthority to SimpleGrantedAuthority, but the problem remained.

My User class looks like follows:

public class User implements UserDetails {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;
    private String fName;
    private String lName;
    private String phone;
    private String email;

    // Properties for Spring authentication

    @ElementCollection(targetClass=SimpleGrantedAuthority.class, fetch = FetchType.EAGER)
    @JsonDeserialize(as = SimpleGrantedAuthority.class)
    private Set<? extends GrantedAuthority> grantedAuthorities;
    @NotNull
    private String password;
    @NotNull
    private String username;
    private boolean isAccountNonExpired;
    private boolean isAccountNonLocked;
    private boolean isCredentialsNonExpired;
    private boolean isEnabled;

    public User(String fName, String lName, String phone, String email, 
                Set<SimpleGrantedAuthority> grantedAuthorities, String password, String username, 
                boolean isAccountNonExpired, boolean isAccountNonLocked, 
                boolean isCredentialsNonExpired, boolean isEnabled) {       
        this.fName = fName;
        this.lName = lName;
        this.phone = phone;
        this.email = email;
        this.grantedAuthorities = grantedAuthorities;
        this.password = password;
        this.username = username;
        this.isAccountNonExpired = isAccountNonExpired;
        this.isAccountNonLocked = isAccountNonLocked;
        this.isCredentialsNonExpired = isCredentialsNonExpired;
        this.isEnabled = isEnabled;
    }
    // Getters, setters & constructors
}

My UserController`s method to create a new user:

@RestController
public class UserController {
    @Autowired
    UserService userService;

    @PostMapping("/users")
    public User createNew(@Valid @RequestBody User user) {
        return userService.addUser(user);
    }
}

Which calls the UserService:

public class UserService implements UserDetailsService {

    @Autowired
    UserRepository userRepo;

    public User addUser(User user) {

        Set<SimpleGrantedAuthority> userRoles = user.getGrantedAuthorities()
                .stream()
                .map(p -> new SimpleGrantedAuthority(p.getAuthority()))
                .collect(Collectors.toSet());

        User newUser = new User(user.getfName(), user.getlName(), 
                user.getPhone(), user.getEmail(), 
                userRoles, 
                user.getPassword(), user.getUsername(), 
                user.isAccountNonExpired(), 
                user.isAccountNonLocked(), 
                user.isCredentialsNonExpired(), 
                user.isEnabled());

        return userRepo.save(newUser);
    }
}

Now, posting a JSON like this works fine:

{
    "fName": "First",
    "lName": "Last",
    "phone": "+3111111111",
    "email": "test@mail.com",
    "password": "password",
    "username": "user",
    "enabled": true,
    "authorities": [],
    "accountNonLocked": true,
    "accountNonExpired": true,
    "credentialsNonExpired": true
}

However, adding some kind of authority will cause the error mentioned above:

{
    "fName": "First",
    "lName": "Last",
    "phone": "+3111111111",
    "email": "test@mail.com",
    "password": "password",
    "username": "user",
    "enabled": true,
    "authorities": [{
            "authority": "inventory:read"
        }],
   "accountNonLocked": true,
    "accountNonExpired": true,
    "credentialsNonExpired": true
}

Any kind of help is appreciated!