Error: *.belongsToMany called with something that's not a subclass of Sequelize.Model code example

Example 1: belongsTo called with something that's not a subclass of Sequelize.Model.

Foto.belongsTo(User, {foreignKey: 'userId'});

Example 2: belongsTo called with something that's not a subclass of Sequelize.Model.

User.hasMany(Foto,{as: 'fotos', foreignKey: 'userId'})