diff --git a/exrtools/src/pngoutput.c b/exrtools/src/pngoutput.c

index e08f4a6..54ad812 100644

--- a/exrtools/src/pngoutput.c

+++ b/exrtools/src/pngoutput.c

@@ -27,6 +27,7 @@

 #include <errno.h>

 #include <string.h>

 #include <png.h>

+#include <zlib.h>

 #include "pngoutput.h"

 struct pngoutput_s