diff --git a/apps/openmw/mwclass/creature.cpp b/apps/openmw/mwclass/creature.cpp
index 6646a7d79..2483f2a7e 100644
--- a/apps/openmw/mwclass/creature.cpp
+++ b/apps/openmw/mwclass/creature.cpp
@@ -448,7 +448,7 @@ namespace MWClass
         return ref->mBase->mScript;
     }
 
-    bool Creature::isEssential (const MWWorld::Ptr& ptr) const
+    bool Creature::isEssential (const MWWorld::ConstPtr& ptr) const
     {
         return isFlagBitSet(ptr, ESM::Creature::Essential);
     }
diff --git a/apps/openmw/mwclass/creature.hpp b/apps/openmw/mwclass/creature.hpp
index 720d87f17..72305a1d8 100644
--- a/apps/openmw/mwclass/creature.hpp
+++ b/apps/openmw/mwclass/creature.hpp
@@ -83,7 +83,7 @@ namespace MWClass
             virtual float getArmorRating (const MWWorld::Ptr& ptr) const;
             ///< @return combined armor rating of this actor
 
-            virtual bool isEssential (const MWWorld::Ptr& ptr) const;
+            virtual bool isEssential (const MWWorld::ConstPtr& ptr) const;
             ///< Is \a ptr essential? (i.e. may losing \a ptr make the game unwinnable)
 
             virtual int getServices (const MWWorld::Ptr& actor) const;
diff --git a/apps/openmw/mwclass/npc.cpp b/apps/openmw/mwclass/npc.cpp
index 350b644ed..0e9618608 100644
--- a/apps/openmw/mwclass/npc.cpp
+++ b/apps/openmw/mwclass/npc.cpp
@@ -908,10 +908,9 @@ namespace MWClass
         return ptr.getRefData().getCustomData()->asNpcCustomData().mMovement;
     }
 
-    bool Npc::isEssential (const MWWorld::Ptr& ptr) const
+    bool Npc::isEssential (const MWWorld::ConstPtr& ptr) const
     {
-        MWWorld::LiveCellRef<ESM::NPC> *ref =
-            ptr.get<ESM::NPC>();
+        const MWWorld::LiveCellRef<ESM::NPC> *ref = ptr.get<ESM::NPC>();
 
         return (ref->mBase->mFlags & ESM::NPC::Essential) != 0;
     }
diff --git a/apps/openmw/mwclass/npc.hpp b/apps/openmw/mwclass/npc.hpp
index 684d891f9..3dfb0b96e 100644
--- a/apps/openmw/mwclass/npc.hpp
+++ b/apps/openmw/mwclass/npc.hpp
@@ -114,7 +114,7 @@ namespace MWClass
             virtual void skillUsageSucceeded (const MWWorld::Ptr& ptr, int skill, int usageType, float extraFactor=1.f) const;
             ///< Inform actor \a ptr that a skill use has succeeded.
 
-            virtual bool isEssential (const MWWorld::Ptr& ptr) const;
+            virtual bool isEssential (const MWWorld::ConstPtr& ptr) const;
             ///< Is \a ptr essential? (i.e. may losing \a ptr make the game unwinnable)
 
             virtual int getServices (const MWWorld::Ptr& actor) const;
diff --git a/apps/openmw/mwworld/class.cpp b/apps/openmw/mwworld/class.cpp
index a0037df57..a07c94a0c 100644
--- a/apps/openmw/mwworld/class.cpp
+++ b/apps/openmw/mwworld/class.cpp
@@ -218,7 +218,7 @@ namespace MWWorld
         throw std::runtime_error ("encumbrance not supported by class");
     }
 
-    bool Class::isEssential (const MWWorld::Ptr& ptr) const
+    bool Class::isEssential (const MWWorld::ConstPtr& ptr) const
     {
         return false;
     }
diff --git a/apps/openmw/mwworld/class.hpp b/apps/openmw/mwworld/class.hpp
index 04d22f91a..4fc0c96ec 100644
--- a/apps/openmw/mwworld/class.hpp
+++ b/apps/openmw/mwworld/class.hpp
@@ -229,7 +229,7 @@ namespace MWWorld
             ///
             /// (default implementations: throws an exception)
 
-            virtual bool isEssential (const MWWorld::Ptr& ptr) const;
+            virtual bool isEssential (const MWWorld::ConstPtr& ptr) const;
             ///< Is \a ptr essential? (i.e. may losing \a ptr make the game unwinnable)
             ///
             /// (default implementation: return false)