diff --git a/Source/JavaScriptCore/heap/HandleSet.cpp b/Source/JavaScriptCore/heap/HandleSet.cpp
index 70e35901cbdbc..383e009b67208 100644
--- a/Source/JavaScriptCore/heap/HandleSet.cpp
+++ b/Source/JavaScriptCore/heap/HandleSet.cpp
@@ -29,6 +29,7 @@
 #include "HandleBlock.h"
 #include "HandleBlockInlines.h"
 #include "JSCJSValueInlines.h"
+#include "VM.h"
 
 namespace JSC {
 
@@ -93,4 +94,17 @@ bool HandleSet::isLiveNode(Node* node)
 }
 #endif // ENABLE(GC_VALIDATION) || ASSERT_ENABLED
 
+#if ASSERT_ENABLED
+void HandleSet::assertMayMutate()
+{
+    // Allocating, deallocating, or re-targeting a Strong handle mutates
+    // m_strongList, which the GC's strong-handles marking constraint scans.
+    // The VM's API lock is what orders those mutations with the scan, so a
+    // mutation from a thread that does not hold the lock is a data race. The
+    // classic way to get here is a Strong captured by value in a lambda that
+    // another thread destroys.
+    ASSERT_WITH_MESSAGE(m_vm.currentThreadIsHoldingAPILock(), "Strong handles may only be created, written, or destroyed while holding their VM's API lock");
+}
+#endif // ASSERT_ENABLED
+
 } // namespace JSC
diff --git a/Source/JavaScriptCore/heap/HandleSet.h b/Source/JavaScriptCore/heap/HandleSet.h
index 25819fc16540e..661e3cf99ea65 100644
--- a/Source/JavaScriptCore/heap/HandleSet.h
+++ b/Source/JavaScriptCore/heap/HandleSet.h
@@ -86,6 +86,12 @@ class HandleSet {
     JS_EXPORT_PRIVATE bool isLiveNode(Node*);
 #endif
 
+#if ASSERT_ENABLED
+    JS_EXPORT_PRIVATE void assertMayMutate();
+#else
+    void assertMayMutate() { }
+#endif
+
     VM& m_vm;
     DoublyLinkedList<HandleBlock> m_blockList;
 
@@ -106,6 +112,7 @@ inline VM& HandleSet::vm()
 
 inline HandleSlot HandleSet::allocate()
 {
+    assertMayMutate();
     if (m_freeList.isEmpty())
         grow();
 
@@ -116,6 +123,7 @@ inline HandleSlot HandleSet::allocate()
 
 inline void HandleSet::deallocate(HandleSlot handle)
 {
+    assertMayMutate();
     HandleSet::Node* node = HandleNode::toHandleNode(handle);
     if (node->isOnList())
         NodeList::remove(node);
@@ -147,6 +155,7 @@ template<typename Functor> void HandleSet::forEachStrongHandle(const Functor& fu
 template<bool isCellOnly>
 inline void HandleSet::writeBarrier(HandleSlot slot, JSValue value)
 {
+    assertMayMutate();
     bool valueIsNonEmptyCell = value && (isCellOnly || value.isCell());
     bool slotIsNonEmptyCell = *slot && (isCellOnly || slot->isCell());
     if (valueIsNonEmptyCell == slotIsNonEmptyCell)