diff --git a/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py b/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py
index d5855a755..a4abef966 100644
--- a/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py
+++ b/test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py
@@ -41,11 +41,16 @@ def serialize(obj: SSZValue):
     if isinstance(obj, BasicValue):
         return serialize_basic(obj)
     elif isinstance(obj, Bitvector):
-        as_integer = sum([obj[i] << i for i in range(len(obj))])
-        return as_integer.to_bytes((len(obj) + 7) // 8, "little")
+        as_bytearray = [0] * ((len(obj) + 7) // 8)
+        for i in range(len(obj)):
+            as_bytearray[i // 8] |= obj[i] << (i % 8)
+        return bytes(as_bytearray)
     elif isinstance(obj, Bitlist):
-        as_integer = (1 << len(obj)) + sum([obj[i] << i for i in range(len(obj))])
-        return as_integer.to_bytes((as_integer.bit_length() + 7) // 8, "little")
+        as_bytearray = [0] * (len(obj) // 8 + 1)
+        for i in range(len(obj)):
+            as_bytearray[i // 8] |= obj[i] << (i % 8)
+        as_bytearray[len(obj) // 8] |= 1 << (len(obj) % 8)
+        return bytes(as_bytearray)
     elif isinstance(obj, Series):
         return encode_series(obj)
     else:
@@ -92,12 +97,11 @@ def encode_series(values: Series):
 def pack(values: Series):
     if isinstance(values, bytes):  # Bytes and BytesN are already packed
         return values
-    elif isinstance(values, Bitvector):
-        as_integer = sum([values[i] << i for i in range(len(values))])
-        return as_integer.to_bytes((values.length + 7) // 8, "little")
-    elif isinstance(values, Bitlist):
-        as_integer = sum([values[i] << i for i in range(len(values))])
-        return as_integer.to_bytes((values.length + 7) // 8, "little")
+    elif isinstance(values, Bitvector) or isinstance(values, Bitlist):
+        as_bytearray = [0] * ((len(values) + 7) // 8)
+        for i in range(len(values)):
+            as_bytearray[i // 8] |= values[i] << (i % 8)
+        return bytes(as_bytearray)
     return b''.join([serialize_basic(value) for value in values])