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