https://jsfiddle.net/3apqnhmw/

```jsx
class Foo extends React.Component {
  render() {
     return <div>Foo</div>
  }
}

class App extends React.Component {
  render() {
    return Foo
  }
}
```

When `App` returns `Foo` instead of `<Foo />` accidentally, Fiber does not catch this.
);
    }
    if (isArray(newChild)) {
      return reconcileChildrenArray(
        returnFiber,
        currentFirstChild,
        newChild,
        priority,
      );
    }
    if (getIteratorFn(newChild)) {
      return reconcileChildrenIterator(
        returnFiber,
        currentFirstChild,
        newChild,
        priority,
      );
    }
    if (isObject) {
      throwOnInvalidObjectType(returnFiber, newChild);
    }
    if (!disableNewFiberFeatures && typeof newChild === 'undefined') {
      // 
      // 
      // 
      switch (returnFiber.tag) {
        case ClassComponent: {
          if (__DEV__) {
            const instance = returnFiber.stateNode;
            if (instance.render._isMockFunction) {