Why does Postgres think a number value is null?

I'm adding this object to a Postgres table:

[ { subjects_id: 6 }, { user_id: 52 } ]

and Postgres thinks the user_id is null. Why? As you can see below, user_id was set to null, but it was certainly changed because console.log(tutorSubjectRelation) logs user_id with a number as shown above.

This is the code that's attempting to insert in a table the above object using a method:

 const user = []
        const tutorSubjectRelation = [{subjects_id:null}, {user_id:null}]
        UsersService.insertUser(
            req.app.get('db'),
            newUser
        )
            .then(res => {
                user.push(res)
                tutorSubjectRelation[1].user_id = user[0].user_id
                console.log(user)
                return SubjectsService.getBySubject(
                    req.app.get('db'),
                    subjects
                )
            })
            .then(res => {
                if (typeof res === 'undefined') {

                    return SubjectsService.insertSubject(
                        req.app.get('db'),
                        subjects
                    )
                }
                return res
            })
            .then(res => {
                tutorSubjectRelation[0].subjects_id = res.subject_id
                console.log(tutorSubjectRelation);
                return TutorsSubjectsService.insertTutorSubject(
                    req.app.get('db'),
                    tutorSubjectRelation
                )
            }) const user = []
        const tutorSubjectRelation = [{subjects_id:null}, {user_id:null}]
        UsersService.insertUser(
            req.app.get('db'),
            newUser
        )
            .then(res => {
                user.push(res)
                tutorSubjectRelation[1].user_id = user[0].user_id
                console.log(user)
                return SubjectsService.getBySubject(
                    req.app.get('db'),
                    subjects
                )
            })
            .then(res => {
                if (typeof res === 'undefined') {
                    return SubjectsService.insertSubject(
                        req.app.get('db'),
                        subjects
                    )
                }
                return res
            })
            .then(res => {
                tutorSubjectRelation[0].subjects_id = res.subject_id
                console.log(tutorSubjectRelation);
                return TutorsSubjectsService.insertTutorSubject(
                    req.app.get('db'),
                    tutorSubjectRelation
                )

            })

and I am getting this error in Postman:

{
    "message": "insert into \"tutors_subjects\" (\"subjects_id\", \"user_id\") values ($1, DEFAULT), (DEFAULT, $2) returning * - null value in column \"user_id\" violates not-null constraint",

Update: the issue was that I was entering one row as two rows; as pointed out in the comments. Thank you!