SequelizeUniqueConstraintError: Validation error

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?

ERROR: SequelizeUniqueConstraintError: Validation error

this is my code :

up: (queryInterface, Sequelize) => {
    return Promise.all([
      queryInterface.changeColumn('PersonalInfo', 'ssn', {type: Sequelize.STRING, unique: true
      }),
      queryInterface.changeColumn('PersonalInfoDraft', 'ssn', {type: Sequelize.STRING, unique:true
      })
    ])
  }