diff --git a/src/tsearch.h b/src/tsearch.h
index 7ba71d97502ca34908284560d0c156d2d29d9c37..7b297db7c7ea425f350f0d2c3350d55a630fb97b 100644
--- a/src/tsearch.h
+++ b/src/tsearch.h
@@ -20,7 +20,6 @@ void *tfind(const void *vkey, void * const *vrootp,
             int (*compar)(const void *, const void *));
 
 #else
-#define _POSIX_C_SOURCE 200809L
 #include <search.h>
 #endif