--- a/psi/iapi.h
+++ b/psi/iapi.h
@@ -313,6 +313,7 @@
     int argc, char **argv);
 
 #ifdef __WIN32__
+#include <stddef.h>
 GSDLLEXPORT int GSDLLAPI gsapi_init_with_argsA(void *instance,
     int argc, char **argv);
 
