diff --git a/clash_royale/envs/game_engine/card.py b/clash_royale/envs/game_engine/card.py
index 82e744e..cc7d5d6 100644
--- a/clash_royale/envs/game_engine/card.py
+++ b/clash_royale/envs/game_engine/card.py
@@ -5,6 +5,7 @@ class Card():
     This class is created for Player class to refer to statistics of cards.
     '''
 
-    def __init__(self, elixir: int) -> None:
+    def __init__(self, elixir: int, elixir_cost: int) -> None:
         self.elixir: int = elixir
+        self.elixir_cost: int = elixir_cost
         pass
diff --git a/clash_royale/envs/game_engine/player.py b/clash_royale/envs/game_engine/player.py
index 5bc54a0..4e93872 100644
--- a/clash_royale/envs/game_engine/player.py
+++ b/clash_royale/envs/game_engine/player.py
@@ -76,19 +76,17 @@ def pop(self, card_index: int) -> None:
 
         self.deck.put(self.hand[card_index], block = False)
         self.hand[card_index] = self.next
-        self.next: Card = self.deck.get(block = False)
-
+        self.next = self.deck.get(block = False)
 
     def play_card(self, card_index: int) -> None:
 
         """
-        
-        Called with the index of cards in hand to update the state of available cards, next cards, and elixir.
+        Called with the index of cards in hand to update the state of 
+        available cards, next cards, and elixir.
         Should be called with the initial rendering of entity.
-
         """
 
-        assert(card_index < 4)
+        assert card_index >= 0 and card_index < 4
         elixir_cost: float = self.hand[card_index].elixir_cost
         assert(elixir_cost <= self.elixir)
         self.pop(card_index)