--- a/src/vasprintf.c
+++ b/src/vasprintf.c
@@ -71,6 +71,7 @@ If not, see <https://www.gnu.org/license
 #endif /* HAVE_VA_COPY */
 
 #ifdef HAVE_WCHAR_H
+#include <stddef.h>
 #include <wchar.h>
 #endif