diff --git a/lib/asan/asan_mac.cc b/lib/asan/asan_mac.cc
index 9200837..f436f5d 100644
--- a/lib/asan/asan_mac.cc
+++ b/lib/asan/asan_mac.cc
@@ -386,7 +386,7 @@ INTERCEPTOR(void, dispatch_group_async_f, dispatch_group_t group,
                                asan_dispatch_call_block_and_release);
 }
 
-#if !defined(MISSING_BLOCKS_SUPPORT)
+#if defined(__BLOCKS__)
 extern "C" {
 // FIXME: consolidate these declarations with asan_intercepted_functions.h.
 void dispatch_async(dispatch_queue_t dq, void(^work)(void));