Of $COURSE you can't redefine constants :)

lib/datalib.php

@@ -20,7 +20,6 @@ if ($SITE = get_site()) {
      * If $SITE global from {@link get_site()} is set then SITEID to $SITE->id, otherwise set to 1.
      */
     define('SITEID', $SITE->id);
-    define('COURSEID', SITEID);   // For now.  This will usually get reset later in require_login() etc.
 } else {
     /**
      * @ignore
@@ -29,6 +28,8 @@ if ($SITE = get_site()) {
     define('COURSEID', 1);
 }
 
+$COURSE->id = SITEID;   // For now.  This will usually get reset later in require_login() etc.
+
 /// FUNCTIONS FOR DATABASE HANDLING  ////////////////////////////////
 
 /**