diff --git a/packages/react-reconciler/src/ReactFiberHooks.new.js b/packages/react-reconciler/src/ReactFiberHooks.new.js
index 2f854410afb11..110431001349f 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.new.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.new.js
@@ -46,6 +46,7 @@ import {
 import {
   REACT_CONTEXT_TYPE,
   REACT_SERVER_CONTEXT_TYPE,
+  REACT_MEMO_CACHE_SENTINEL,
 } from 'shared/ReactSymbols';
 
 import {
@@ -845,8 +846,6 @@ function useMemoCache(size: number): Array<any> {
     memoCache = updateQueue.memoCache;
   }
   // Otherwise clone from the current fiber
-  // TODO: not sure how to access the current fiber here other than going through
-  // currentlyRenderingFiber.alternate
   if (memoCache == null) {
     const current: Fiber | null = currentlyRenderingFiber.alternate;
     if (current !== null) {
@@ -878,6 +877,9 @@ function useMemoCache(size: number): Array<any> {
   let data = memoCache.data[memoCache.index];
   if (data === undefined) {
     data = memoCache.data[memoCache.index] = new Array(size);
+    for (let i = 0; i < size; i++) {
+      data[i] = REACT_MEMO_CACHE_SENTINEL;
+    }
   } else if (data.length !== size) {
     // TODO: consider warning or throwing here
     if (__DEV__) {
diff --git a/packages/react-reconciler/src/ReactFiberHooks.old.js b/packages/react-reconciler/src/ReactFiberHooks.old.js
index bbdb1e2478f99..527d442f88d22 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.old.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.old.js
@@ -46,6 +46,7 @@ import {
 import {
   REACT_CONTEXT_TYPE,
   REACT_SERVER_CONTEXT_TYPE,
+  REACT_MEMO_CACHE_SENTINEL,
 } from 'shared/ReactSymbols';
 
 import {
@@ -845,8 +846,6 @@ function useMemoCache(size: number): Array<any> {
     memoCache = updateQueue.memoCache;
   }
   // Otherwise clone from the current fiber
-  // TODO: not sure how to access the current fiber here other than going through
-  // currentlyRenderingFiber.alternate
   if (memoCache == null) {
     const current: Fiber | null = currentlyRenderingFiber.alternate;
     if (current !== null) {
@@ -878,6 +877,9 @@ function useMemoCache(size: number): Array<any> {
   let data = memoCache.data[memoCache.index];
   if (data === undefined) {
     data = memoCache.data[memoCache.index] = new Array(size);
+    for (let i = 0; i < size; i++) {
+      data[i] = REACT_MEMO_CACHE_SENTINEL;
+    }
   } else if (data.length !== size) {
     // TODO: consider warning or throwing here
     if (__DEV__) {
diff --git a/packages/react-reconciler/src/__tests__/useMemoCache-test.js b/packages/react-reconciler/src/__tests__/useMemoCache-test.js
index 94684ec8c00b3..bde6671fc5ac5 100644
--- a/packages/react-reconciler/src/__tests__/useMemoCache-test.js
+++ b/packages/react-reconciler/src/__tests__/useMemoCache-test.js
@@ -1,8 +1,19 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ * @jest-environment node
+ */
+
 let React;
 let ReactNoop;
 let act;
 let useState;
 let useMemoCache;
+let MemoCacheSentinel;
 let ErrorBoundary;
 
 describe('useMemoCache()', () => {
@@ -14,6 +25,7 @@ describe('useMemoCache()', () => {
     act = require('jest-react').act;
     useState = React.useState;
     useMemoCache = React.unstable_useMemoCache;
+    MemoCacheSentinel = Symbol.for('react.memo_cache_sentinel');
 
     class _ErrorBoundary extends React.Component {
       constructor(props) {
@@ -46,7 +58,8 @@ describe('useMemoCache()', () => {
       const cache = useMemoCache(1);
       expect(Array.isArray(cache)).toBe(true);
       expect(cache.length).toBe(1);
-      expect(cache[0]).toBe(undefined);
+      expect(cache[0]).toBe(MemoCacheSentinel);
+
       return 'Ok';
     }
     const root = ReactNoop.createRoot();
diff --git a/packages/react-server/src/ReactFizzHooks.js b/packages/react-server/src/ReactFizzHooks.js
index bd0be42d14563..5d5eca52319de 100644
--- a/packages/react-server/src/ReactFizzHooks.js
+++ b/packages/react-server/src/ReactFizzHooks.js
@@ -46,6 +46,7 @@ import is from 'shared/objectIs';
 import {
   REACT_SERVER_CONTEXT_TYPE,
   REACT_CONTEXT_TYPE,
+  REACT_MEMO_CACHE_SENTINEL,
 } from 'shared/ReactSymbols';
 
 type BasicStateAction<S> = (S => S) | S;
@@ -666,7 +667,11 @@ function useCacheRefresh(): <T>(?() => T, ?T) => void {
 }
 
 function useMemoCache(size: number): Array<any> {
-  return new Array(size);
+  const data = new Array(size);
+  for (let i = 0; i < size; i++) {
+    data[i] = REACT_MEMO_CACHE_SENTINEL;
+  }
+  return data;
 }
 
 function noop(): void {}
diff --git a/packages/react-server/src/ReactFlightHooks.js b/packages/react-server/src/ReactFlightHooks.js
index a25a5f4b162ae..f4ee8d384d38b 100644
--- a/packages/react-server/src/ReactFlightHooks.js
+++ b/packages/react-server/src/ReactFlightHooks.js
@@ -11,7 +11,10 @@ import type {Dispatcher} from 'react-reconciler/src/ReactInternalTypes';
 import type {Request} from './ReactFlightServer';
 import type {ReactServerContext, Thenable, Usable} from 'shared/ReactTypes';
 import type {ThenableState} from './ReactFlightWakeable';
-import {REACT_SERVER_CONTEXT_TYPE} from 'shared/ReactSymbols';
+import {
+  REACT_SERVER_CONTEXT_TYPE,
+  REACT_MEMO_CACHE_SENTINEL,
+} from 'shared/ReactSymbols';
 import {readContext as readContextImpl} from './ReactFlightNewContext';
 import {enableUseHook} from 'shared/ReactFeatureFlags';
 import {
@@ -90,7 +93,11 @@ export const HooksDispatcher: Dispatcher = {
     return unsupportedRefresh;
   },
   useMemoCache(size: number): Array<any> {
-    return new Array(size);
+    const data = new Array(size);
+    for (let i = 0; i < size; i++) {
+      data[i] = REACT_MEMO_CACHE_SENTINEL;
+    }
+    return data;
   },
   use: enableUseHook ? use : (unsupportedHook: any),
 };
diff --git a/packages/shared/ReactSymbols.js b/packages/shared/ReactSymbols.js
index c68e50d5c693e..74bb4f0de92d9 100644
--- a/packages/shared/ReactSymbols.js
+++ b/packages/shared/ReactSymbols.js
@@ -45,6 +45,10 @@ export const REACT_SERVER_CONTEXT_DEFAULT_VALUE_NOT_LOADED: symbol = Symbol.for(
   'react.default_value',
 );
 
+export const REACT_MEMO_CACHE_SENTINEL: symbol = Symbol.for(
+  'react.memo_cache_sentinel',
+);
+
 const MAYBE_ITERATOR_SYMBOL = Symbol.iterator;
 const FAUX_ITERATOR_SYMBOL = '@@iterator';