title: SequelizeUniqueConstraintError: Validation error
body: <p>I am using sequelize and node.js. I have one column that want change it's type to unique, when I want run the migration I have this err , How can I solve it?</p>
  <pre><code>ERROR: SequelizeUniqueConstraintError: Validation error
  </code></pre>
  <p>this is my code :</p>
  <pre><code>
  up: (queryInterface, Sequelize) =&gt; {
      return Promise.all([
        queryInterface.changeColumn('PersonalInfo', 'ssn', {type: Sequelize.STRING, unique: true
        }),
        queryInterface.changeColumn('PersonalInfoDraft', 'ssn', {type: Sequelize.STRING, unique:true
        })
      ])
    }
  </code></pre>