diff --git a/include/mimalloc-internal.h b/include/mimalloc-internal.h
index 06d31dbd..9c898c4a 100644
--- a/include/mimalloc-internal.h
+++ b/include/mimalloc-internal.h
@@ -831,7 +831,7 @@ static inline void* mi_tls_slot(size_t slot) mi_attr_noexcept {
   res = tcb[slot];
 #elif defined(__aarch64__)
   void** tcb; UNUSED(ofs);
-#if defined(__APPLE__) // issue #343
+#if defined(__APPLE__) // M1, issue #343
   __asm__ volatile ("mrs %0, tpidrro_el0" : "=r" (tcb));
 #else
   __asm__ volatile ("mrs %0, tpidr_el0" : "=r" (tcb));
@@ -858,7 +858,7 @@ static inline void mi_tls_slot_set(size_t slot, void* value) mi_attr_noexcept {
   tcb[slot] = value;
 #elif defined(__aarch64__)
   void** tcb; UNUSED(ofs);
-#if defined(__APPLE__) // issue #343
+#if defined(__APPLE__) // M1, issue #343
   __asm__ volatile ("mrs %0, tpidrro_el0" : "=r" (tcb));
 #else
   __asm__ volatile ("mrs %0, tpidr_el0" : "=r" (tcb));
@@ -868,8 +868,13 @@ static inline void mi_tls_slot_set(size_t slot, void* value) mi_attr_noexcept {
 }
 
 static inline uintptr_t _mi_thread_id(void) mi_attr_noexcept {
-  // in all our targets, slot 0 is the pointer to the thread control block
+#if defined(__aarch64__) && defined(__APPLE__)  // M1
+  // on macOS on the M1, slot 0 does not seem to work, so we fall back to portable C for now. See issue #354
+  return (uintptr_t)&_mi_heap_default;
+#else
+  // in all our other targets, slot 0 is the pointer to the thread control block
   return (uintptr_t)mi_tls_slot(0);
+#endif
 }
 #else
 // otherwise use standard C
diff --git a/src/heap.c b/src/heap.c
index c5195491..274e6b51 100644
--- a/src/heap.c
+++ b/src/heap.c
@@ -359,7 +359,9 @@ static void mi_heap_absorb(mi_heap_t* heap, mi_heap_t* from) {
   // turns out to be ok as `_mi_heap_delayed_free` only visits the list and calls a 
   // the regular `_mi_free_delayed_block` which is safe.
   _mi_heap_delayed_free(from);  
+  #if !defined(_MSC_VER) || (_MSC_VER > 1900) // somehow the following line gives an error in VS2015, issue #353
   mi_assert_internal(mi_atomic_load_ptr_relaxed(mi_block_t,&from->thread_delayed_free) == NULL);
+  #endif
 
   // and reset the `from` heap
   mi_heap_reset_pages(from);