diff --git a/ariths_gen/multi_bit_circuits/others/popcount.py b/ariths_gen/multi_bit_circuits/others/popcount.py
index 04fcda3..4721bce 100644
--- a/ariths_gen/multi_bit_circuits/others/popcount.py
+++ b/ariths_gen/multi_bit_circuits/others/popcount.py
@@ -1,7 +1,7 @@
 """
 
 """
-from typing import Union
+from typing import Union, Optional
 
 from ariths_gen.wire_components import (
     Wire,
@@ -45,7 +45,7 @@ class UnsignedPopCount(GeneralCircuit):
 
     """
 
-    def __init__(self, a: Bus, adder : Union[ArithmeticCircuit, None] = None, prefix : str = "", name : str = "popcnt", **kwargs):
+    def __init__(self, a: Bus, adder : Optional[ArithmeticCircuit] = None, prefix : str = "", name : str = "popcnt", **kwargs):
         self.N = a.N
         self.a = a
 
diff --git a/tests/test_popcnt.py b/tests/test_popcnt.py
index 52de63f..a877254 100644
--- a/tests/test_popcnt.py
+++ b/tests/test_popcnt.py
@@ -1,3 +1,4 @@
+from io import StringIO
 from ariths_gen.core.cgp_circuit import UnsignedCGPCircuit
 from ariths_gen.wire_components import (
     Wire,
@@ -11,6 +12,7 @@ from ariths_gen.core.arithmetic_circuits import GeneralCircuit
 from ariths_gen.multi_bit_circuits.others import (
     UnsignedPopCount
 )
+import numpy as np
 
 
 def test_popcount():