Warning accessing protected properties in extern base class

Here's a reduction of the problem:

Externs:

``` js
/** @constructor */
var Foo = function() {};
/** @protected */
Foo.prototype.doIt = function() {};
```

Main Module:

``` js
/**
* @extends {Foo}
* @constructor
*/
var Bar = function() {};

/** @this {Bar} */
var doThing = function() {
this.doIt();
};
```

This produces the warning:
WARNING - Access to protected property doIt of Bar not allowed here.
this.doIt();

Some Notes:
This doesn't occur if I move the base class into the main module.
This doesn't occur if I use Bar.prototype.doThing instead of the @this annotation.