diff --git a/packages/react-devtools-shared/src/devtools/store.js b/packages/react-devtools-shared/src/devtools/store.js
index 4c9131c896785..b51597f43a528 100644
--- a/packages/react-devtools-shared/src/devtools/store.js
+++ b/packages/react-devtools-shared/src/devtools/store.js
@@ -681,6 +681,7 @@ export default class Store extends EventEmitter<{
             let depth = 0;
             while (parentID > 0) {
               if (parentID === ownerID || unsortedIDs.has(parentID)) {
+                // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
                 depth = depthMap.get(parentID) + 1;
                 depthMap.set(id, depth);
                 break;
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js b/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js
index 1103cc0f6c830..e975bd16fab63 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js
@@ -505,6 +505,7 @@ function reduceSearchState(store: Store, state: State, action: Action): State {
         if (numPrevSearchResults > 0) {
           didRequestSearch = true;
           searchIndex =
+            // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
             searchIndex + 1 < numPrevSearchResults ? searchIndex + 1 : 0;
         }
         break;
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js b/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js
index 1d4fd41d798a7..faf124b5c97ae 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js
@@ -126,7 +126,10 @@ export default function SnapshotSelector(_: Props) {
         type="text"
         inputMode="numeric"
         pattern="[0-9]*"
-        value={selectedFilteredCommitIndex + 1}
+        value={
+          // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
+          selectedFilteredCommitIndex + 1
+        }
         size={`${numFilteredCommits}`.length}
         onChange={handleCommitInputChange}
         onClick={handleClick}
diff --git a/packages/react-reconciler/src/ReactFiberCompleteWork.new.js b/packages/react-reconciler/src/ReactFiberCompleteWork.new.js
index 135e9290f549f..592bef4fbd25d 100644
--- a/packages/react-reconciler/src/ReactFiberCompleteWork.new.js
+++ b/packages/react-reconciler/src/ReactFiberCompleteWork.new.js
@@ -662,8 +662,10 @@ function bubbleProperties(completedWork: Fiber) {
         // this value will reflect the amount of time spent working on a previous
         // render. In that case it should not bubble. We determine whether it was
         // cloned by comparing the child pointer.
+        // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
         actualDuration += child.actualDuration;
 
+        // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
         treeBaseDuration += child.treeBaseDuration;
         child = child.sibling;
       }
@@ -712,6 +714,7 @@ function bubbleProperties(completedWork: Fiber) {
         subtreeFlags |= child.subtreeFlags & StaticMask;
         subtreeFlags |= child.flags & StaticMask;
 
+        // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
         treeBaseDuration += child.treeBaseDuration;
         child = child.sibling;
       }
diff --git a/packages/react-reconciler/src/ReactFiberCompleteWork.old.js b/packages/react-reconciler/src/ReactFiberCompleteWork.old.js
index 3fd1bffbc3a99..f3ba7bf0799a0 100644
--- a/packages/react-reconciler/src/ReactFiberCompleteWork.old.js
+++ b/packages/react-reconciler/src/ReactFiberCompleteWork.old.js
@@ -662,8 +662,10 @@ function bubbleProperties(completedWork: Fiber) {
         // this value will reflect the amount of time spent working on a previous
         // render. In that case it should not bubble. We determine whether it was
         // cloned by comparing the child pointer.
+        // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
         actualDuration += child.actualDuration;
 
+        // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
         treeBaseDuration += child.treeBaseDuration;
         child = child.sibling;
       }
@@ -712,6 +714,7 @@ function bubbleProperties(completedWork: Fiber) {
         subtreeFlags |= child.subtreeFlags & StaticMask;
         subtreeFlags |= child.flags & StaticMask;
 
+        // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
         treeBaseDuration += child.treeBaseDuration;
         child = child.sibling;
       }
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index a40b5ac480728..5e9edf7945653 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -2217,6 +2217,7 @@ function completeUnitOfWork(unitOfWork: Fiber): void {
         let actualDuration = completedWork.actualDuration;
         let child = completedWork.child;
         while (child !== null) {
+          // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
           actualDuration += child.actualDuration;
           child = child.sibling;
         }
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index 3db85b0b9b23b..d8a0bc883f4d7 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -2217,6 +2217,7 @@ function completeUnitOfWork(unitOfWork: Fiber): void {
         let actualDuration = completedWork.actualDuration;
         let child = completedWork.child;
         while (child !== null) {
+          // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
           actualDuration += child.actualDuration;
           child = child.sibling;
         }
diff --git a/packages/react-reconciler/src/ReactProfilerTimer.new.js b/packages/react-reconciler/src/ReactProfilerTimer.new.js
index a5a14eae1c244..de88e711cbf76 100644
--- a/packages/react-reconciler/src/ReactProfilerTimer.new.js
+++ b/packages/react-reconciler/src/ReactProfilerTimer.new.js
@@ -122,6 +122,7 @@ function stopProfilerTimerIfRunningAndRecordDelta(
 
   if (profilerStartTime >= 0) {
     const elapsedTime = now() - profilerStartTime;
+    // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
     fiber.actualDuration += elapsedTime;
     if (overrideBaseTime) {
       fiber.selfBaseDuration = elapsedTime;
@@ -215,6 +216,7 @@ function transferActualDuration(fiber: Fiber): void {
   // where we should count the work of multiple passes.
   let child = fiber.child;
   while (child) {
+    // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
     fiber.actualDuration += child.actualDuration;
     child = child.sibling;
   }
diff --git a/packages/react-reconciler/src/ReactProfilerTimer.old.js b/packages/react-reconciler/src/ReactProfilerTimer.old.js
index a5a14eae1c244..de88e711cbf76 100644
--- a/packages/react-reconciler/src/ReactProfilerTimer.old.js
+++ b/packages/react-reconciler/src/ReactProfilerTimer.old.js
@@ -122,6 +122,7 @@ function stopProfilerTimerIfRunningAndRecordDelta(
 
   if (profilerStartTime >= 0) {
     const elapsedTime = now() - profilerStartTime;
+    // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
     fiber.actualDuration += elapsedTime;
     if (overrideBaseTime) {
       fiber.selfBaseDuration = elapsedTime;
@@ -215,6 +216,7 @@ function transferActualDuration(fiber: Fiber): void {
   // where we should count the work of multiple passes.
   let child = fiber.child;
   while (child) {
+    // $FlowFixMe[unsafe-addition] addition with possible null/undefined value
     fiber.actualDuration += child.actualDuration;
     child = child.sibling;
   }
diff --git a/scripts/flow/config/flowconfig b/scripts/flow/config/flowconfig
index 30ecf58e7e67d..9b47998da9880 100644
--- a/scripts/flow/config/flowconfig
+++ b/scripts/flow/config/flowconfig
@@ -38,10 +38,6 @@
 [lints]
 untyped-type-import=error
 
-# TODO: We might want to enable this this as a error (the default), it
-#       disallows adding undefined and numbers.
-unsafe-addition=off
-
 [options]
 server.max_workers=4
 esproposal.class_static_fields=enable