diff --git a/src/inventory.cpp b/src/inventory.cpp
index e952426dec376..f33ac6e1d30b0 100644
--- a/src/inventory.cpp
+++ b/src/inventory.cpp
@@ -978,3 +978,9 @@ const itype_bin &inventory::get_binned_items() const
     binned = true;
     return binned_items;
 }
+
+void inventory::copy_invlet_of( const inventory &other )
+{
+    assigned_invlet = other.assigned_invlet;
+    invlet_cache = other.invlet_cache;
+}
diff --git a/src/inventory.h b/src/inventory.h
index cee11c954833a..f39022c929fbe 100644
--- a/src/inventory.h
+++ b/src/inventory.h
@@ -178,6 +178,8 @@ class inventory : public visitable<inventory>
 
         void update_cache_with_item( item &newit );
 
+        void copy_invlet_of( const inventory &other );
+
     private:
         // For each item ID, store a set of "favorite" inventory letters.
         std::map<std::string, std::vector<char> > invlet_cache;
diff --git a/src/npctalk.cpp b/src/npctalk.cpp
index 6c0c9cebe88dc..7c022aaea6ac9 100644
--- a/src/npctalk.cpp
+++ b/src/npctalk.cpp
@@ -3735,6 +3735,7 @@ inventory inventory_exchange( inventory &inv,
     inv.dump( item_dump );
     item_dump.insert( item_dump.end(), added.begin(), added.end() );
     inventory new_inv;
+    new_inv.copy_invlet_of( inv );
 
     for( item *it : item_dump ) {
         if( without.count( it ) == 0 ) {