diff --git a/simgear/sound/readwav.cxx b/simgear/sound/readwav.cxx
index 54acda5a..84fd81b9 100644
--- a/simgear/sound/readwav.cxx
+++ b/simgear/sound/readwav.cxx
@@ -21,6 +21,7 @@
 #include "readwav.hxx"
 
 #include <cassert>
+#include <cstdlib>
 
 #include <zlib.h> // for gzXXX functions