diff --git a/model.py b/model.py
index 111a5cb..16c8fb7 100644
--- a/model.py
+++ b/model.py
@@ -6,6 +6,7 @@ def __init__(self, row_num, column_num, paths, units, kings, cells):
         self.row_num = row_num
         self.column_num = column_num
         self.paths = paths
+        self.paths_dict = dict([(path.id, path) for path in paths])
         self.units = units
         self.kings = kings
         self.cells = cells
@@ -19,10 +20,7 @@ def clear_units(self):
                 cell.clear_units()
 
     def get_path_by_id(self, path_id):
-        for path in self.paths:
-            if path.path_id == path_id:
-                return path
-        return None
+        return self.paths_dict[path_id]
 
     def add_unit_in_cell(self, row, column, unit):
         self.cells[row][column].add_unit(unit)
diff --git a/world.py b/world.py
index 63fb2d4..e9c4be5 100644
--- a/world.py
+++ b/world.py
@@ -420,11 +420,17 @@ def get_current_turn(self):
 
     # put unit_id in path_id in position 'index' all spells of one kind have the same id
     def cast_unit_spell(self, unit_id, path_id, index, spell=None, spell_id=None):
+        if spell is None and spell_id is None:
+            return None
         path = None
         for p in self.map.paths:
             if p.path_id == path_id:
                 path = p
                 break
+        if path is None:
+            return None
+        if index < len(path.cells):
+            return None
         cell = path.cells[index]
         if spell is None:
             spell = self.get_spell_by_type_id(spell_id)