code: function(value) {
  
  		var reason = null;
  
  		if (typeof(value) == 'undefined') {
  
  			reason = "Undefined is not a valid value";
  
  		} else if (value == null && !this._is_nullable) {
  
  			reason = "Cannot assign null to non-nullable field";
  
  		}
  
  		return reason;
  
  	}
docs: Unlike {@link Lava.data.field.Abstract#isValidValue}, this is slow version of validity check,
  which returns a message in case the value is invalid
  @param {*} value
  @returns {?string}