diff --git a/src/fallback_malloc.cpp b/src/fallback_malloc.cpp
index 336ad31..bec9952 100644
--- a/src/fallback_malloc.cpp
+++ b/src/fallback_malloc.cpp
@@ -7,6 +7,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// Define _LIBCPP_BUILDING_LIBRARY to ensure _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+// is only defined when libc aligned allocation is not available.
+#define _LIBCPP_BUILDING_LIBRARY
 #include "fallback_malloc.h"
 
 #include <__threading_support>