diff --git a/src/debug.c b/src/debug.c
index 1f48dc9..f9eccad 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -1071,7 +1071,9 @@
 				case O_NOCTTY:  chunk_appendf(&trash, ",O_NOCTTY");  break;
 				case O_TRUNC:   chunk_appendf(&trash, ",O_TRUNC");   break;
 				case O_APPEND:  chunk_appendf(&trash, ",O_APPEND");  break;
+#ifdef O_ASYNC
 				case O_ASYNC:   chunk_appendf(&trash, ",O_ASYNC");   break;
+#endif
 #ifdef O_DIRECT
 				case O_DIRECT:  chunk_appendf(&trash, ",O_DIRECT");  break;
 #endif