Got problem when create a new starting location

I created a map with all wall around and a metal bar door (locked), but when I set the starting location at this map, the player will always be spawn at another map. After I leave a block of wall empty or add a common door, even a reinforced glass shutter, it went normal. What is wrong with this? What kind of terrain may be the valid one for starting?

Search here - Cataclysm-DDA/start_location.cpp at 0469910665608fda5a5e05b50626206e80b31b66 · CleverRaven/Cataclysm-DDA · GitHub