problems creating stub tree

Hello,
I'm trying to integrate StubTrees for fast access to some data of my psi-elements. I implemented everything according to the documentation here: http://confluence.jetbrains.com/display/IDEADEV/Indexing+and+PSI+Stubs+in+IntelliJ+IDEA .
My problem is that my PSI-elements are never backed by stubs.
In my implementation of IStubElementType the createStub- the serialize-, and deserialize-Methods are called. But the createPsi-Method is never called and the getStub-Method always returns null.

Here is the source of my very simple test language plugin:

public interface ItemStub extends StubElement<Item>{
    List<String> getTokens();
}

public class ItemStubImpl extends StubBase<Item> implements ItemStub {

    private List<String> tokens;
    public ItemStubImpl(StubElement parent, List<String> tokens) {
        super(parent, SimpleTypes.ITEM);
        this.tokens = tokens;
    }

    @Override
    public List<String> getTokens() {
        return tokens;
    }

    public void setTokens(List<String> tokens) {
        this.tokens = tokens;
    }
}

public class ItemStubElementImpl <T extends StubElement> extends StubBasedPsiElementBase<T> {
    public ItemStubElementImpl(@NotNull T stub, @NotNull IStubElementType nodeType) {
        super(stub, nodeType);
    }

    public ItemStubElementImpl(@NotNull ASTNode node) {
        super(node);
    }
}

public interface Item extends StubBasedPsiElement<ItemStub> {
    List<String> getTokens();
}

public class ItemImpl extends ItemStubElementImpl<ItemStub> implements Item{
    public ItemImpl(final @NotNull ItemStub stub, final @NotNull IStubElementType nodeType) {
        super(stub, nodeType);
    }

    public ItemImpl(@NotNull ASTNode node) {
        super(node);
    }

    public List<String> getTokens(){
        ItemStub stub = getStub();
        if(stub != null) {
            // unfortunately I never reach this point. My stub is always null
            System.out.println("loaded from stub yeah");
            List<String> tokens = stub.getTokens();
            return tokens;
        }
        ASTNode[] children = getNode().getChildren(TokenSet.create(SimpleTypes.TOKEN));
        List<String> tokens = new ArrayList<String>();
        for (ASTNode child : children) {
            String text = child.getText();
            tokens.add(text);
        }
        return tokens;
    }
}

public class SimpleElementType extends IStubElementType<ItemStub, Item> {
    public SimpleElementType(@NotNull @NonNls String debugName) {
        super(debugName, SimpleLanguage.INSTANCE);
    }

    @Override
    public Item createPsi(@NotNull ItemStub stub) {
        return new ItemImpl(stub, this);
    }

    @Override
    public ItemStub createStub(@NotNull Item psi, StubElement parentStub) {
        // this is called for every Item  
        List<String> tokens = psi.getTokens();
        ItemStubImpl itemStub = new ItemStubImpl(parentStub, tokens);
        return itemStub;
    }

    @Override
    public String getExternalId() {
        return null;
    }

    @Override
    public void serialize(ItemStub stub, StubOutputStream dataStream) throws IOException {
        // this is called
        List<String> tokens = stub.getTokens();
        String join = StringUtils.join(tokens, ",");
        dataStream.writeName(join);
    }

    @Override
    public ItemStub deserialize(StubInputStream dataStream, StubElement parentStub) throws IOException {
        // this is called
        StringRef stringRef = dataStream.readName();
        if(stringRef == null) {
            return null;
        }
        String string = stringRef.getString();
        String[] split = string.split(",");
        List<String> tokens = new ArrayList<String>();
        for (String s : split) {
            tokens.add(s);
        }
        ItemStubImpl itemStub = new ItemStubImpl(parentStub, tokens);
        return itemStub;
    }

    @Override
    public void indexStub(ItemStub stub, IndexSink sink) {
        // TODO later
    }
}

Sorry to just offer the source and hoping that someone finds the error, but I'm not able to find any hints in the web, so I would be very thankful if someone could put me in the right direction,
Sebastian