fix off-by-one in furniture stockpile export

fixes #5557

Author: ab9rf

docs/changelog.txt

@@ -59,6 +59,7 @@ Template for new versions:
 
 ## Fixes
 - ``Units::getReadableName`` will no longer append a comma to the names of histfigs with no profession
+- `stockpiles`: fixed off-by-one error in exporting furniture stockpiles
 
 ## Misc Improvements
 

plugins/stockpiles/StockpileSerializer.cpp

@@ -1741,7 +1741,7 @@ bool StockpileSettingsSerializer::write_furniture(color_ostream& out, StockpileS
             all = false;
             continue;
         }
-        string f_type(type_traits::key_table[i]);
+        string f_type{ENUM_KEY_STR(furniture_type, furniture_type(i))};
         furniture->add_type(f_type);
         DEBUG(log, out).print("furniture_type %zd is %s\n", i, f_type.c_str());
     }