diff --git a/lib/unicodeio.c b/lib/unicodeio.c
index c753e3d50..3565dedd7 100644
--- a/lib/unicodeio.c
+++ b/lib/unicodeio.c
@@ -38,6 +38,9 @@
 #ifndef errno
 extern int errno;
 #endif
+#ifndef ENOTSUP
+# define ENOTSUP EINVAL
+#endif
 
 #if HAVE_LIMITS_H
 # include <limits.h>