diff --git a/associate.mm b/associate.mm
index 9cee7b8f..8a3e4e55 100644
--- a/associate.mm
+++ b/associate.mm
@@ -54,6 +54,11 @@
 	 * @syncronize().
 	 */
 	mutex_t lock;
+	/**
+	 * Lock guarding the structure of the list.  Only the first reference list
+	 * in a chain uses it.
+	 */
+	ThinLock structureLock;
 	/**
 	 * Array of references.
 	 */
@@ -135,12 +140,14 @@ static void setReference(struct reference_list *list,
 		case OBJC_ASSOCIATION_ASSIGN:
 			break;
 	}
-	// While inserting into the list, we need to lock it temporarily.
+	// While inserting into the list, we need to lock it temporarily.  An
+	// existing reference is updated in place.
 	struct reference *r = findReference(list, key);
+	if (NULL == r)
 	{
-		auto lock = acquire_locks_for_pointers(list);
-		// If there's an existing reference, then we can update it, otherwise we
-		// have to install a new one
+		std::lock_guard<ThinLock> lock{list->structureLock};
+		// Another thread may have installed this key since the search above.
+		r = findReference(list, key);
 		if (NULL == r)
 		{
 			// Search for an unused slot