Object.prototype is not an object?

``` js
// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @output_file_name default.js
// ==/ClosureCompiler==

/** @param {Object} x */
function f(x) {
return x.__proto__ == Object.prototype
}
```

I expect no warning, but I get:

```
JSC_DETERMINISTIC_TEST: condition always evaluates to false
left : (Object|null)
right: Object.prototype at line 3 character 9
return x.__proto__ == Object.prototype
```

I think i know why this happens, but need to think more about the solution.