diff --git a/doc/fixes34.4 b/doc/fixes34.4
index d7ba2e465..9d7bad83e 100644
--- a/doc/fixes34.4
+++ b/doc/fixes34.4
@@ -271,6 +271,7 @@ prevent very large number of objects in # inventory slot from causing
 	buffer overflow
 !fixinv config was using arbitrary characters instead of # for invent overflow
 don't report death by petrification if cockatrice kills hero via HP loss
+Riders are immune to green slime
 
 
 Platform- and/or Interface-Specific Fixes
diff --git a/src/mon.c b/src/mon.c
index b205d1f26..f9ad7902a 100644
--- a/src/mon.c
+++ b/src/mon.c
@@ -2584,6 +2584,9 @@ boolean msg;		/* "The oldmon turns into a newmon!" */
 	struct permonst *olddata = mtmp->data;
 	char oldname[BUFSZ], newname[BUFSZ];
 
+	/* Riders are immune to polymorph and green slime */
+	if (is_rider(mtmp->data)) return 0;
+
 	if (msg) {
 	    /* like Monnam() but never mention saddle */
 	    Strcpy(oldname, x_monnam(mtmp, ARTICLE_THE, (char *)0,