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!