diff --git a/GameMap.py b/GameMap.py
index b325ab1..3c8ea47 100644
--- a/GameMap.py
+++ b/GameMap.py
@@ -18,7 +18,7 @@ class GameMap:
     
     
     def add_player_mark(self, x, y, player):
-        if x <= self.max_size_x and y <= self.max_size_y:
+        if (x <= self.max_size_x and y <= self.max_size_y) and (x >= 0 and y >= 0):
             self.data[(x,y)] = player
             player.record_mark(x, y)
         else: