diff --git a/stl/inc/xthreads.h b/stl/inc/xthreads.h
index 8eddb38e0f3..e0c05e459c5 100644
--- a/stl/inc/xthreads.h
+++ b/stl/inc/xthreads.h
@@ -32,20 +32,27 @@ struct _Stl_critical_section {
     _Smtx_t _M_srw_lock = nullptr;
 };
 
+#if defined(_CRT_WINDOWS) || defined(UNDOCKED_WINDOWS_UCRT)
+#pragma detect_mismatch("windows_mutex", "1")
+#else // ^^^ Windows private STL / public STL vvv
+#pragma detect_mismatch("windows_mutex", "0")
+#endif // ^^^ public STL ^^^
+
 struct _Mtx_internal_imp_t {
-#ifdef _CRT_WINDOWS
+#if defined(_CRT_WINDOWS) || defined(UNDOCKED_WINDOWS_UCRT)
 #ifdef _WIN64
     static constexpr size_t _Critical_section_size = 16;
 #else // ^^^ defined(_WIN64) / !defined(_WIN64) vvv
     static constexpr size_t _Critical_section_size = 8;
 #endif // ^^^ !defined(_WIN64) ^^^
-#else // ^^^ defined(_CRT_WINDOWS) / !defined(_CRT_WINDOWS) vvv
+#else // ^^^ Windows private STL / public STL vvv
 #ifdef _WIN64
     static constexpr size_t _Critical_section_size = 64;
 #else // ^^^ defined(_WIN64) / !defined(_WIN64) vvv
     static constexpr size_t _Critical_section_size = 36;
 #endif // ^^^ !defined(_WIN64) ^^^
-#endif // ^^^ !defined(_CRT_WINDOWS) ^^^
+#endif // ^^^ public STL ^^^
+
 
     static constexpr size_t _Critical_section_align = alignof(void*);