diff --git a/Source/WTF/ChangeLog b/Source/WTF/ChangeLog
index 9051f213137f..28793a59cef0 100644
--- a/Source/WTF/ChangeLog
+++ b/Source/WTF/ChangeLog
@@ -1,3 +1,15 @@
+2021-08-18  Ryosuke Niwa  <rniwa@webkit.org>
+
+        [ iOS Debug ] 12 TestWebKitAPI.WebKitLegacy. tests are crashing
+        https://bugs.webkit.org/show_bug.cgi?id=229250
+
+        Reviewed by Wenson Hsieh.
+
+        Disable the assertion when Web Thread is enabled for now.
+
+        * wtf/CheckedRef.h:
+        (WTF::SingleThreadIntegralWrapper::assertThread const):
+
 2021-08-18  Megan Gardner  <megan_gardner@apple.com>
 
         Remove unneeded UIKitSPI declarations
diff --git a/Source/WTF/wtf/CheckedRef.h b/Source/WTF/wtf/CheckedRef.h
index 393f1238a65f..d2bc1cace332 100644
--- a/Source/WTF/wtf/CheckedRef.h
+++ b/Source/WTF/wtf/CheckedRef.h
@@ -248,10 +248,14 @@ class SingleThreadIntegralWrapper {
     SingleThreadIntegralWrapper& operator--();
 
 private:
+#if ASSERT_ENABLED && !USE(WEB_THREAD)
     void assertThread() const { ASSERT(m_thread.ptr() == &Thread::current()); }
+#else
+    constexpr void assertThread() const { }
+#endif
 
     IntegralType m_value;
-#if ASSERT_ENABLED
+#if ASSERT_ENABLED && !USE(WEB_THREAD)
     Ref<Thread> m_thread;
 #endif
 };
@@ -259,7 +263,7 @@ class SingleThreadIntegralWrapper {
 template <typename IntegralType>
 inline SingleThreadIntegralWrapper<IntegralType>::SingleThreadIntegralWrapper(IntegralType value)
     : m_value { value }
-#if ASSERT_ENABLED
+#if ASSERT_ENABLED && !USE(WEB_THREAD)
     , m_thread { Thread::current() }
 #endif
 { }