diff --git a/scripts/fiber/tests-failing.txt b/scripts/fiber/tests-failing.txt
index 36f8480baf..2316f076e1 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 18c4945ca2..0c7d4082c5 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 61efde7c9b..ea7f39da29 100644
--- a/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
+++ b/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
@@ -1015,8 +1015,10 @@ describe('ReactDOMFiber', () => {
       });
 
       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);
@@ -1030,9 +1032,9 @@ describe('ReactDOMFiber', () => {
           return props.children;
         }
 
-        expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(/Render\.render/);
-        expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(/Render\.render/);
-        expect(() => ReactDOM.render(<Render>[<div />]</Render>, container)).toThrow(/Render\.render/);
+        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 acaaff8725..c741d94bdd 100644
--- a/src/renderers/shared/fiber/ReactChildFiber.js
+++ b/src/renderers/shared/fiber/ReactChildFiber.js
@@ -75,6 +75,7 @@ const {
   NoEffect,
   Placement,
   Deletion,
+  Err,
 } = ReactTypeOfSideEffect;
 
 function coerceRef(current: ?Fiber, element: ReactElement) {
@@ -1129,7 +1130,9 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       if (returnFiber.tag === HostRoot) {
         // Top-level only accepts elements or portals
         invariant(
-          false,
+          // 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 {
@@ -1247,7 +1250,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           const Component = returnFiber.type;
           invariant(
             false,
-            '%s: Nothing was returned from render. This usually means a ' +
+            '%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'
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js b/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
index e356056af4..cb0b5e2472 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 bee26adef8..603ab8c7fd 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 a095ee0ca4..876f268554 100644
--- a/src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
+++ b/src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
@@ -146,8 +146,13 @@ describe('ReactStatelessComponent', () => {
     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.'
+      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.'
     );
   });