directives scope with optional attributes?

[johnnyelwailer]

Hi
I have a directive with its own scope.

scope: {optionalAttribute: '='}

I want the attributes to be optional.

if (scope.optionalAttribute == null)
{
scope.optionalAttribute= 'unnamed';
}

http://jsfiddle.net/johnnayweiler/28fJP/

This gives me the 'Non-assignable model expression'error.
Why is that so and how to enable default values for those attributes(make them optional)?

Thanks.

[pkozlowski-opensource]

Closing as duplicate of #1435