diff --git a/scripts/fiber/tests-failing.txt b/scripts/fiber/tests-failing.txt
index 36f8480baf3..2316f076e13 100644
--- a/scripts/fiber/tests-failing.txt
+++ b/scripts/fiber/tests-failing.txt
@@ -64,10 +64,6 @@ src/renderers/shared/shared/__tests__/ReactEmptyComponent-test.js
 src/renderers/shared/shared/__tests__/ReactMultiChildText-test.js
 * should reorder keyed text nodes
 
-src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
-* should warn when stateless component returns array
-* should warn when using non-React functions in JSX
-
 src/renderers/shared/shared/__tests__/ReactUpdates-test.js
 * marks top-level updates
 
diff --git a/scripts/fiber/tests-passing.txt b/scripts/fiber/tests-passing.txt
index 18c4945ca2a..0c7d4082c56 100644
--- a/scripts/fiber/tests-passing.txt
+++ b/scripts/fiber/tests-passing.txt
@@ -544,6 +544,8 @@ src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
 * should bubble events from the portal to the parent
 * should not onMouseLeave when staying in the portal
 * should not crash encountering low-priority tree
+* throws if non-element passed to top-level render
+* throws if something other than false, null, or an element is returned from render
 
 src/renderers/dom/shared/__tests__/CSSProperty-test.js
 * should generate browser prefixes for its `isUnitlessNumber`
@@ -1536,6 +1538,7 @@ src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
 * should update stateless component
 * should unmount stateless component
 * should pass context thru stateless component
+* should throw when stateless component returns undefined
 * should throw on string refs in pure functions
 * should warn when given a string ref
 * should warn when given a function ref
diff --git a/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js b/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
index 9abec3eedd1..ea7f39da297 100644
--- a/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
+++ b/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
@@ -1002,5 +1002,40 @@ describe('ReactDOMFiber', () => {
         container
       );
     });
+
+    describe('disableNewFiberFeatures', () => {
+      var ReactFeatureFlags = require('ReactFeatureFlags');
+
+      beforeEach(() => {
+        ReactFeatureFlags.disableNewFiberFeatures = true;
+      });
+
+      afterEach(() => {
+        ReactFeatureFlags.disableNewFiberFeatures = false;
+      });
+
+      it('throws if non-element passed to top-level render', () => {
+        // FIXME: These assertions pass individually, but they leave React in
+        // an inconsistent state. This suggests an error-handling bug. I'll fix
+        // this in a separate PR.
+        const message = 'render(): Invalid component element.';
+        expect(() => ReactDOM.render(null, container)).toThrow(message, container);
+        expect(() => ReactDOM.render(undefined, container)).toThrow(message, container);
+        expect(() => ReactDOM.render(false, container)).toThrow(message, container);
+        expect(() => ReactDOM.render('Hi', container)).toThrow(message, container);
+        expect(() => ReactDOM.render(999, container)).toThrow(message, container);
+        expect(() => ReactDOM.render([<div />], container)).toThrow(message, container);
+      });
+
+      it('throws if something other than false, null, or an element is returned from render', () => {
+        function Render(props) {
+          return props.children;
+        }
+
+        expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(/You may have returned undefined/);
+        expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(/You may have returned undefined/);
+        expect(() => ReactDOM.render(<Render>[<div />]</Render>, container)).toThrow(/You may have returned undefined/);
+      });
+    });
   }
 });
diff --git a/src/renderers/shared/fiber/ReactChildFiber.js b/src/renderers/shared/fiber/ReactChildFiber.js
index 9302c2ed809..c741d94bddf 100644
--- a/src/renderers/shared/fiber/ReactChildFiber.js
+++ b/src/renderers/shared/fiber/ReactChildFiber.js
@@ -36,6 +36,7 @@ var ReactTypeOfWork = require('ReactTypeOfWork');
 var emptyObject = require('emptyObject');
 var getIteratorFn = require('getIteratorFn');
 var invariant = require('invariant');
+var ReactFeatureFlags = require('ReactFeatureFlags');
 
 if (__DEV__) {
   var { getCurrentFiberStackAddendum } = require('ReactDebugCurrentFiber');
@@ -60,8 +61,10 @@ const {
 const isArray = Array.isArray;
 
 const {
+  FunctionalComponent,
   ClassComponent,
   HostText,
+  HostRoot,
   HostPortal,
   CoroutineComponent,
   YieldComponent,
@@ -72,6 +75,7 @@ const {
   NoEffect,
   Placement,
   Deletion,
+  Err,
 } = ReactTypeOfSideEffect;
 
 function coerceRef(current: ?Fiber, element: ReactElement) {
@@ -1100,6 +1104,68 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     // not as a fragment. Nested arrays on the other hand will be treated as
     // fragment nodes. Recursion happens at the normal flow.
 
+    if (ReactFeatureFlags.disableNewFiberFeatures) {
+      // Support only the subset of return types that Stack supports. Treat
+      // everything else as empty, but log a warning.
+      if (typeof newChild === 'object' && newChild !== null) {
+        switch (newChild.$$typeof) {
+          case REACT_ELEMENT_TYPE:
+            return placeSingleChild(reconcileSingleElement(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+
+          case REACT_PORTAL_TYPE:
+            return placeSingleChild(reconcileSinglePortal(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+        }
+      }
+
+      if (returnFiber.tag === HostRoot) {
+        // Top-level only accepts elements or portals
+        invariant(
+          // If the root has an error effect, this is an intentional unmount.
+          // Don't throw an error.
+          returnFiber.effectTag & Err,
+          'render(): Invalid component element.'
+        );
+      } else {
+        switch (returnFiber.tag) {
+          case ClassComponent: {
+            if (__DEV__) {
+              const instance = returnFiber.stateNode;
+              if (instance.render._isMockFunction) {
+                // We allow auto-mocks to proceed as if they're
+                // returning null.
+                break;
+              }
+            }
+          }
+          // Intentionally fall through to the next case, which handles both
+          // functions and classes
+          // eslint-disable-next-lined no-fallthrough
+          case FunctionalComponent: {
+            // Composites accept elements, portals, null, or false
+            const Component = returnFiber.type;
+            invariant(
+              newChild === null || newChild === false,
+              '%s.render(): A valid React element (or null) must be ' +
+              'returned. You may have returned undefined, an array or some ' +
+              'other invalid object.',
+              Component.displayName || Component.name || 'Component'
+            );
+          }
+        }
+      }
+      return deleteRemainingChildren(returnFiber, currentFirstChild);
+    }
+
     if (typeof newChild === 'string' || typeof newChild === 'number') {
       return placeSingleChild(reconcileSingleTextNode(
         returnFiber,
@@ -1163,6 +1229,36 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       }
     }
 
+    if (typeof newChild === 'undefined') {
+      switch (returnFiber.tag) {
+        case HostRoot:
+          // TODO: Top-level render
+          break;
+        case ClassComponent: {
+          if (__DEV__) {
+            const instance = returnFiber.stateNode;
+            if (instance.render._isMockFunction) {
+              // We allow auto-mocks to proceed as if they're returning null.
+              break;
+            }
+          }
+        }
+        // Intentionally fall through to the next case, which handles both
+        // functions and classes
+        // eslint-disable-next-lined no-fallthrough
+        case FunctionalComponent: {
+          const Component = returnFiber.type;
+          invariant(
+            false,
+            '%s(...): Nothing was returned from render. This usually means a ' +
+            'return statement is missing. Or, to render nothing, ' +
+            'return null.',
+            Component.displayName || Component.name || 'Component'
+          );
+        }
+      }
+    }
+
     // Remaining cases are all treated as empty.
     return deleteRemainingChildren(returnFiber, currentFirstChild);
   }
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js b/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
index e356056af4c..cb0b5e24720 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
@@ -956,6 +956,7 @@ describe('ReactIncremental', () => {
 
     function Trail() {
       ops.push('Trail');
+      return null;
     }
 
     function App(props) {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
index bee26adef8f..603ab8c7fd2 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
@@ -496,6 +496,7 @@ describe('ReactIncrementalErrorHandling', () => {
       if (props.throw) {
         throw new Error('Hello');
       }
+      return null;
     }
 
     function Foo() {
diff --git a/src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js b/src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
index 060ec461373..876f2685548 100644
--- a/src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
+++ b/src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
@@ -15,6 +15,8 @@ var React;
 var ReactDOM;
 var ReactTestUtils;
 
+var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
+
 function StatelessComponent(props) {
   return <div>{props.name}</div>;
 }
@@ -123,18 +125,34 @@ describe('ReactStatelessComponent', () => {
     );
   });
 
-  it('should warn when stateless component returns array', () => {
-    spyOn(console, 'error');
+  if (!ReactDOMFeatureFlags.useFiber) {
+    // Stack doesn't support fragments
+    it('should throw when stateless component returns array', () => {
+      function NotAComponent() {
+        return [<div />, <div />];
+      }
+      expect(function() {
+        ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
+      }).toThrowError(
+        'NotAComponent(...): A valid React element (or null) must be returned. ' +
+        'You may have returned undefined, an array or some other invalid object.'
+      );
+    });
+  }
+
+  it('should throw when stateless component returns undefined', () => {
     function NotAComponent() {
-      return [<div />, <div />];
     }
     expect(function() {
       ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
-    }).toThrow();
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
-      'NotAComponent(...): A valid React element (or null) must be returned. ' +
-      'You may have returned undefined, an array or some other invalid object.'
+    }).toThrowError(
+      ReactDOMFeatureFlags.useFiber ?
+        // Fiber gives a more specific error message for undefined because it
+        // supports more return types.
+        'NotAComponent(...): Nothing was returned from render' :
+        // Stack's message is generic.
+        'NotAComponent(...): A valid React element (or null) must be returned. ' +
+        'You may have returned undefined, an array or some other invalid object.'
     );
   });
 
@@ -374,18 +392,4 @@ describe('ReactStatelessComponent', () => {
     expect(() => ReactTestUtils.renderIntoDocument(<Child />)).not.toThrow();
   });
 
-  it('should warn when using non-React functions in JSX', () => {
-    spyOn(console, 'error');
-    function NotAComponent() {
-      return [<div />, <div />];
-    }
-    expect(function() {
-      ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
-    }).toThrow();  // has no method 'render'
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
-      'NotAComponent(...): A valid React element (or null) must be returned. You may ' +
-      'have returned undefined, an array or some other invalid object.'
-    );
-  });
 });
diff --git a/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js b/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js
index 1abe88d6224..ecdccd09824 100644
--- a/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js
+++ b/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js
@@ -39,26 +39,9 @@ function StatelessComponent(Component) {
 StatelessComponent.prototype.render = function() {
   var Component = ReactInstanceMap.get(this)._currentElement.type;
   var element = Component(this.props, this.context, this.updater);
-  warnIfInvalidElement(Component, element);
   return element;
 };
 
-function warnIfInvalidElement(Component, element) {
-  if (__DEV__) {
-    warning(
-      element === null || element === false || React.isValidElement(element),
-      '%s(...): A valid React element (or null) must be returned. You may have ' +
-      'returned undefined, an array or some other invalid object.',
-      Component.displayName || Component.name || 'Component'
-    );
-    warning(
-      !Component.childContextTypes,
-      '%s(...): childContextTypes cannot be defined on a functional component.',
-      Component.displayName || Component.name || 'Component'
-    );
-  }
-}
-
 function shouldConstruct(Component) {
   return !!(Component.prototype && Component.prototype.isReactComponent);
 }
@@ -205,7 +188,13 @@ var ReactCompositeComponent = {
     // Support functional components
     if (!doConstruct && (inst == null || inst.render == null)) {
       renderedElement = inst;
-      warnIfInvalidElement(Component, renderedElement);
+      if (__DEV__) {
+        warning(
+          !Component.childContextTypes,
+          '%s(...): childContextTypes cannot be defined on a functional component.',
+          Component.displayName || Component.name || 'Component'
+        );
+      }
       invariant(
         inst === null ||
         inst === false ||
diff --git a/src/renderers/shared/utils/ReactFeatureFlags.js b/src/renderers/shared/utils/ReactFeatureFlags.js
index 5bd9651aed1..ce0a442c11b 100644
--- a/src/renderers/shared/utils/ReactFeatureFlags.js
+++ b/src/renderers/shared/utils/ReactFeatureFlags.js
@@ -18,6 +18,7 @@ var ReactFeatureFlags = {
   // timeline profiles in Chrome, for example.
   logTopLevelRenders: false,
   prepareNewChildrenBeforeUnmountInStack: true,
+  disableNewFiberFeatures: false,
 };
 
 module.exports = ReactFeatureFlags;