bug: Symbol Not Found Error when calling weakReference.stillAlive()

// ResourceCache.mc

import Toybox.Lang;
import Toybox.WatchUi;

class ResourceCache {
    hidden var cache as Dictionary<Symbol, WeakReference> = {} as Dictionary<Symbol, WeakReference>;

    function initialize() {
    }

    function get(symbol as Symbol) as Resource {
        var cache = self.cache;
        var value;
        var weakReference = cache.get(symbol);
        if (weakReference != null && weakReference.stillAlive()) {
            value = weakReference.get() as Resource;
        } else {
            value = WatchUi.loadResource(symbol);
            cache.put(symbol, value.weak());
            logMemory("CACHE: ", ", loaded " + symbol.toString());
        }
        return value;
    }
}

const CACHE = new ResourceCache();

<strings>
    <string id="AppName">Foo</string>

    <string id="foo" translatable="false">FOO</string>
</strings>

class MyApp extends Application.AppBase {

    function initialize() {
        AppBase.initialize();
        CACHE.get(Rez.Strings.foo);
    }
}

I get this error:

Error: Symbol Not Found Error
Details: Could not find symbol 'stillAlive'
Stack: 
  - get() at ResourceCache.mc:14 0x100021b9 
  - initialize() at MyApp.mc:21 0x10003ba4 
  

In the manifest minApiLevel: 1.3.0, so there should be WeakReference.stillAlive.

I tried it in the simulator (SDK 6.4.2) on fr965 and fenix6, in both devices I get the same error.