Creating a new Object returns AttributeError: 'list' object has no attribute '_sa_instance_state'

Hi I am trying to do a simple application where users are capable of creating new cocktails. Therefore I have two models with a many2many relationship

from . import db
assoc_table = db.Table('association',
   db.Column('ingredient_id', db.Integer, db.ForeignKey('ingredients.id')),
   db.Column('cocktail_id', db.Integer, db.ForeignKey('cocktails.id'))
)


class Ingredient(db.Model):

    __tablename__ = 'ingredients'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=True)
    cost_price = db.Column(db.Float, default=0.0)
    cocktails = db.relationship('Cocktail',
                                secondary=assoc_table,
                                backref=db.backref('ingredients'),
                                lazy='dynamic')


class Cocktail(db.Model):

    __tablename__ = 'cocktails'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=True)
    serv_percentage = 0.25
    sell_price = db.Column(db.Float)

And in the view, I send the form if it is a GET request. If it's a POST, I try to create the Cocktail object and then save it on database:

@main.route('/new', methods=['GET', 'POST'])
def new():
    form = CocktailForm()
    form.ingredients.choices = [(i.id, i.name) for i in Ingredient.query.all()]
    if form.validate_on_submit():
        cocktail_name = form.name.data
        cocktail_ingredients = Ingredient.query.filter(Ingredient.id.in_(form.ingredients.data)).all()
        c = Cocktail()
        c.name = form.name.data
        c.ingredients.append(cocktail_ingredients)
        db.session.add(c)
        db.session.commit()
        return redirect(url_for('.index'))
    return render_template('new.html', form=form)

I get AttributeError:

'list' object has no attribute '_sa_instance_state'

in the line that tries to create an empty Cocktail:

c = Cocktail()

I don't have a clue what is the problem.

I have checked many answers and usually is related to relationships so I don't really know what's wrong with my code

Here is the form class, although I think it's not the problem:

class CocktailForm(Form):
    name = StringField('What is the coktail\'s name?', validators=[Required()])
    ingredients = SelectMultipleField('Ingredients', coerce=int)
    submit = SubmitField('Submit')

Thank you