diff --git a/src/ds/defines.h b/src/ds/defines.h
index 69b9106..10bba45 100644
--- a/src/ds/defines.h
+++ b/src/ds/defines.h
@@ -3,6 +3,7 @@
 #if defined(_MSC_VER) && !defined(__clang__)
 // 28 is FAST_FAIL_INVALID_BUFFER_ACCESS.  Not using the symbolic constant to
 // avoid depending on winnt.h
+#  include <intrin.h> // for __fastfail
 #  define SNMALLOC_FAST_FAIL() __fastfail(28)
 #  define ALWAYSINLINE __forceinline
 #  define NOINLINE __declspec(noinline)
@@ -141,32 +142,42 @@ namespace snmalloc
 #  endif
 #endif
 
-inline SNMALLOC_FAST_PATH void check_client_error(const char* const str)
-{
-  //[[clang::musttail]]
-  return snmalloc::error(str);
-}
-
-inline SNMALLOC_FAST_PATH void
-check_client_impl(bool test, const char* const str)
-{
-  if (SNMALLOC_UNLIKELY(!test))
-    check_client_error(str);
-}
-#ifdef SNMALLOC_CHECK_CLIENT
-#  define check_client(test, str) check_client_impl(test, str)
-#else
-#  define check_client(test, str)
-#endif
-
 namespace snmalloc
 {
+  template<typename... Args>
+  SNMALLOC_FAST_PATH_INLINE void UNUSED(Args&&...)
+  {}
+
+  inline SNMALLOC_FAST_PATH void check_client_error(const char* const str)
+  {
+    //[[clang::musttail]]
+    return snmalloc::error(str);
+  }
+
+  inline SNMALLOC_FAST_PATH void
+  check_client_impl(bool test, const char* const str)
+  {
+    if (SNMALLOC_UNLIKELY(!test))
+    {
+#ifdef NDEBUG
+      UNUSED(str);
+      SNMALLOC_FAST_FAIL();
+#else
+      check_client_error(str);
+#endif
+    }
+  }
+
 #ifdef SNMALLOC_CHECK_CLIENT
   static constexpr bool CHECK_CLIENT = true;
 #else
   static constexpr bool CHECK_CLIENT = false;
 #endif
-  template<typename... Args>
-  SNMALLOC_FAST_PATH_INLINE void UNUSED(Args&&...)
-  {}
 } // namespace snmalloc
+
+#ifdef SNMALLOC_CHECK_CLIENT
+#  define snmalloc_check_client(test, str) \
+    snmalloc::check_client_impl(test, str)
+#else
+#  define snmalloc_check_client(test, str)
+#endif
diff --git a/src/mem/corealloc.h b/src/mem/corealloc.h
index ca9937e..89adfc8 100644
--- a/src/mem/corealloc.h
+++ b/src/mem/corealloc.h
@@ -379,7 +379,7 @@ namespace snmalloc
         alloc_classes[sizeclass].unused--;
 
         // TODO delay the clear to the next user of the slab, or teardown so
-        // don't touch the cache lines at this point in check_client.
+        // don't touch the cache lines at this point in snmalloc_check_client.
         auto chunk_record = clear_slab(meta, sizeclass);
         ChunkAllocator::dealloc<SharedStateHandle>(
           get_backend_local_state(),
@@ -664,7 +664,7 @@ namespace snmalloc
 
       SNMALLOC_ASSERT(!meta->is_unused());
 
-      check_client(
+      snmalloc_check_client(
         Metaslab::is_start_of_object(
           entry.get_sizeclass().as_small(), address_cast(p)),
         "Not deallocating start of an object");
diff --git a/src/mem/freelist.h b/src/mem/freelist.h
index 4640534..d2e5a18 100644
--- a/src/mem/freelist.h
+++ b/src/mem/freelist.h
@@ -181,7 +181,7 @@ namespace snmalloc
         void check_prev(address_t signed_prev)
         {
           UNUSED(signed_prev);
-          check_client(
+          snmalloc_check_client(
             signed_prev == this->prev_encoded,
             "Heap corruption - free list corrupted!");
         }
diff --git a/src/mem/localalloc.h b/src/mem/localalloc.h
index 4568f28..b243290 100644
--- a/src/mem/localalloc.h
+++ b/src/mem/localalloc.h
@@ -523,7 +523,7 @@ namespace snmalloc
           metaentry_chunk_sizeclass_to_slab_sizeclass(entry_sizeclass);
 
         // Check for start of allocation.
-        check_client(
+        snmalloc_check_client(
           pointer_align_down(p_tame, size) == p_tame,
           "Not start of an allocation.");
 
@@ -560,7 +560,7 @@ namespace snmalloc
       // If p_tame is not null, then dealloc has been call on something
       // it shouldn't be called on.
       // TODO: Should this be tested even in the !CHECK_CLIENT case?
-      check_client(p_tame == nullptr, "Not allocated by snmalloc.");
+      snmalloc_check_client(p_tame == nullptr, "Not allocated by snmalloc.");
 
 #  ifdef SNMALLOC_TRACING
       std::cout << "nullptr deallocation" << std::endl;