diff --git a/02_hash_maps/hm.py b/02_hash_maps/hm.py
index f807ad8..e7fba61 100644
--- a/02_hash_maps/hm.py
+++ b/02_hash_maps/hm.py
@@ -8,14 +8,19 @@ class HashMap():
 	def __init__(self, hash_size=513):
 		self._hash_size = hash_size
 		self._size = 0
-		self.hmap = [] * self._hash_size
+		self.hmap = [0] * self._hash_size
 		
 	def add(self, value):
 		"""
-		Adds the provided value to the hashmap
+		Adds the provided value to the hashmap.
+		Raises an Exception if a collision is detected
 		"""
 		key = self._hash(value)
-		self.hmap[key] = value
+		if self.hmap[key] == 0:
+			self.hmap[key] = value
+			self._size += 1
+		else: 
+			raise Exception("Collision detected at index %d", key)
 	
 		# TODO: Keep implementing
 	
diff --git a/02_hash_maps/hm_test.py b/02_hash_maps/hm_test.py
index 90c38de..2649bfe 100644
--- a/02_hash_maps/hm_test.py
+++ b/02_hash_maps/hm_test.py
@@ -35,5 +35,11 @@ class test_hash_map(unittest.TestCase):
 		value = ""
 		self.assertRaises(Exception, lambda x : hm._hash(value))
 
+	def test_add(self):
+		hm = HashMap()
+		
+		hm.add("a")
+		self.assertEqual(hm.size(), 1)
+
 if __name__ == "__main__":
 	unittest.main()