The attack target can still be an NPC without being an instance of Player.

src/main/java/com/gmail/nossr50/util/skills/CombatUtils.java

@@ -302,7 +302,7 @@ public final class CombatUtils {
         else if (attacker instanceof Player) {
             Player player = (Player) attacker;
 
-            if (Misc.isNPCEntity(player)) {
+            if (Misc.isNPCEntity(player) || Misc.isNPCEntity(target)) {
                 return;
             }