diff --git a/packages/react-reconciler/src/ReactCapturedValue.js b/packages/react-reconciler/src/ReactCapturedValue.js
index 5f7ca2fcc77e7..f09be0c813325 100644
--- a/packages/react-reconciler/src/ReactCapturedValue.js
+++ b/packages/react-reconciler/src/ReactCapturedValue.js
@@ -13,7 +13,7 @@ import {getStackAddendumByWorkInProgressFiber} from 'shared/ReactFiberComponentT
 
 export type CapturedValue<T> = {
   value: T,
-  source: Fiber | null,
+  source: Fiber,
   stack: string | null,
 };
 
@@ -29,7 +29,7 @@ export type CapturedError = {
 
 export function createCapturedValue<T>(
   value: T,
-  source: Fiber | null,
+  source: Fiber,
 ): CapturedValue<T> {
   // If the value is an error, call this function immediately after it is thrown
   // so the stack is accurate.
diff --git a/packages/react-reconciler/src/ReactFiber.js b/packages/react-reconciler/src/ReactFiber.js
index 720fb9ffe1fb7..84f2fdb239441 100644
--- a/packages/react-reconciler/src/ReactFiber.js
+++ b/packages/react-reconciler/src/ReactFiber.js
@@ -4,10 +4,11 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
+ * @flow
  */
 
 import type {ReactElement, Source} from 'shared/ReactElementType';
-import type {ReactPortal, RefObject} from 'shared/ReactTypes';
+import type {ReactFragment, ReactPortal, RefObject} from 'shared/ReactTypes';
 import type {TypeOfWork} from 'shared/ReactTypeOfWork';
 import type {TypeOfMode} from './ReactTypeOfMode';
 import type {TypeOfSideEffect} from 'shared/ReactTypeOfSideEffect';
@@ -320,7 +321,7 @@ export function createWorkInProgress(
   return workInProgress;
 }
 
-export function createHostRootFiber(isAsync): Fiber {
+export function createHostRootFiber(isAsync: boolean): Fiber {
   const mode = isAsync ? AsyncMode | StrictMode : NoContext;
   return createFiber(HostRoot, null, null, mode);
 }
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.js b/packages/react-reconciler/src/ReactFiberCommitWork.js
index f3184ced83c83..967d90a449c0a 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.js
@@ -9,7 +9,7 @@
 
 import type {HostConfig} from 'react-reconciler';
 import type {Fiber} from './ReactFiber';
-import type {FiberRoot} from './ReactFiber';
+import type {FiberRoot} from './ReactFiberRoot';
 import type {ExpirationTime} from './ReactFiberExpirationTime';
 import type {CapturedValue, CapturedError} from './ReactCapturedValue';
 
@@ -65,7 +65,7 @@ export function logError(boundary: Fiber, errorInfo: CapturedValue<mixed>) {
   }
 
   const capturedError: CapturedError = {
-    componentName: source !== null ? getComponentName(source) : null,
+    componentName: getComponentName(source),
     componentStack: stack !== null ? stack : '',
     error: errorInfo.value,
     errorBoundary: null,
@@ -95,7 +95,7 @@ export function logError(boundary: Fiber, errorInfo: CapturedValue<mixed>) {
 
 export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
   config: HostConfig<T, P, I, TI, HI, PI, C, CC, CX, PL>,
-  captureError: (failedFiber: Fiber, error: mixed) => Fiber | null,
+  captureError: (failedFiber: Fiber, error: mixed) => void,
   scheduleWork: (
     fiber: Fiber,
     startTime: ExpirationTime,
diff --git a/packages/react-reconciler/src/ReactFiberScheduler.js b/packages/react-reconciler/src/ReactFiberScheduler.js
index 562039393da4d..97688243da1dc 100644
--- a/packages/react-reconciler/src/ReactFiberScheduler.js
+++ b/packages/react-reconciler/src/ReactFiberScheduler.js
@@ -321,10 +321,12 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
       }
 
       // Restore the original state of the work-in-progress
-      assignFiberPropertiesInDEV(
-        failedUnitOfWork,
-        stashedWorkInProgressProperties,
-      );
+      if (stashedWorkInProgressProperties !== null) {
+        assignFiberPropertiesInDEV(
+          failedUnitOfWork,
+          stashedWorkInProgressProperties,
+        );
+      }
       switch (failedUnitOfWork.tag) {
         case HostRoot:
           popHostContainer(failedUnitOfWork);
@@ -1078,7 +1080,7 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
         }
 
         if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
-          const failedUnitOfWork = nextUnitOfWork;
+          const failedUnitOfWork: Fiber = nextUnitOfWork;
           replayUnitOfWork(failedUnitOfWork, thrownValue, isAsync);
         }
 
diff --git a/packages/react-reconciler/src/ReactStrictModeWarnings.js b/packages/react-reconciler/src/ReactStrictModeWarnings.js
index 0f39b09371d28..2ac119782e764 100644
--- a/packages/react-reconciler/src/ReactStrictModeWarnings.js
+++ b/packages/react-reconciler/src/ReactStrictModeWarnings.js
@@ -107,7 +107,7 @@ if (__DEV__) {
     pendingUnsafeLifecycleWarnings = new Map();
   };
 
-  const getStrictRoot = (fiber: Fiber): Fiber => {
+  const getStrictRoot = (fiber: Fiber | null): Fiber | null => {
     let maybeStrictRoot = null;
 
     while (fiber !== null) {
@@ -227,6 +227,10 @@ if (__DEV__) {
   ) => {
     const strictRoot = getStrictRoot(fiber);
 
+    if (strictRoot === null) {
+      return;
+    }
+
     // Dedup strategy: Warn once per component.
     // This is difficult to track any other way since component names
     // are often vague and are likely to collide between 3rd party libraries.