diff --git a/src/hb.hh b/src/hb.hh
index 1f1426752..b9f5f7141 100644
--- a/src/hb.hh
+++ b/src/hb.hh
@@ -447,6 +447,7 @@ static int HB_UNUSED _hb_errno = 0;
 #ifndef HB_USE_ATEXIT
 #  define HB_USE_ATEXIT 0
 #endif
+#ifndef hb_atexit
 #if !HB_USE_ATEXIT
 #  define hb_atexit(_) HB_STMT_START { if (0) (_) (); } HB_STMT_END
 #else /* HB_USE_ATEXIT */
@@ -457,6 +458,7 @@ static int HB_UNUSED _hb_errno = 0;
 #    define hb_atexit(f) static hb_atexit_t<f> _hb_atexit_##__LINE__;
 #  endif
 #endif
+#endif
 
 /* Lets assert int types.  Saves trouble down the road. */
 static_assert ((sizeof (hb_codepoint_t) == 4), "");