--- ico.imageio/icooutput.cpp
+++ ico.imageio/icooutput.cpp
@@ -32,6 +32,8 @@
 #include <cstdlib>
 #include <cmath>
 
+#include <zlib.h> /* Z_BEST_COMPRESSION */
+
 #include "ico.h"
 #include "../png.imageio/png_pvt.h"