diff --git a/include/errno.h b/include/errno.h
index 3743eb013..5dba2a1f7 100644
--- a/include/errno.h
+++ b/include/errno.h
@@ -47,8 +47,8 @@ extern unsigned char _oserror;
  */
 int __fastcall__ _osmaperrno (unsigned char oserror);
 
-/* Set errno to a specific error code. Used by the library */
-void __fastcall__ _seterrno (unsigned char code);
+/* Set errno to a specific error code and return zero. Used by the library */
+unsigned char __fastcall__ _seterrno (unsigned char code);
 
 /* System error codes go here */
 extern int _errno;