ng-Messages Syntax Error

[shaohaolin]

I am creating the directive for form field. And I got a ng-messages syntax error on the following code:

The error:

Angular explaination:

Angular version: 1.4.1
Angular message version: 1.4.1

[shaohaolin]

After couple hours researching, I found out the reason.
ng-messages no longer interprets its attribute value properly #11616
(angular/angular.js#11616)

Because angular message 1.4.1 no longer support interprets its attribute value, we can not use {{ }} to evaluate angular express in ng-messages.
The way I walk around with this: