Check if Description of issue was changed.

Hello.
I trying to check if description of issue was changed:
```
guard: function (ctx) {
return (ctx.issue.fields.isChanged(ctx.issue.description) && ctx.issue.isReported);
},
```
but it does't work. I thing it because the description is a property (string) not field. Can I check if the description was changed another way?

Thanks.