How to validate a string field in sequelize.js?

How can I validate a database field to accept just a string?

In my database, I have two fields:

1. description: String
2. completed: Boolean

I want the description field accept just a string value. What I mean is:

• 'description':'text' => database accept this request
• 'descrition': true or false => database refuse this request
• 'descrition': 123 => database refuse this request

Currently, the description field can accept a boolean value , so there is an issue in my configuration.

module.exports = function(sequelize, DataTypes) {
    return sequelize.define('todo', {
        description: {
            type: DataTypes.STRING,
            allowNull:false,
            validate: {
                len: [1, 250],
                isBoolean:false,
                isAlpha:true
            }
        },
        completed: {
            type: DataTypes.BOOLEAN,
            allowNull: false,
            defaultValue: false,
            validate:{
                isBoolean:true
            }
        }
    });
};