diff --git a/lib/spec/runners/ssz_generic.ex b/lib/spec/runners/ssz_generic.ex
index da0963341..28cb8a00b 100644
--- a/lib/spec/runners/ssz_generic.ex
+++ b/lib/spec/runners/ssz_generic.ex
@@ -78,7 +78,7 @@ defmodule SszGenericTestRunner do
     assert serialized == real_serialized
   end
 
-  defp assert_ssz("valid", schema, real_serialized, real_deserialized, hash_tree_root) do
+  defp assert_ssz("valid", schema, real_serialized, real_deserialized, expected_hash_tree_root) do
     {:ok, deserialized} = SszEx.decode(real_serialized, schema)
     assert deserialized == real_deserialized
 
@@ -86,7 +86,9 @@ defmodule SszGenericTestRunner do
 
     assert serialized == real_serialized
 
-    assert hash_tree_root == SszEx.hash_tree_root!(real_deserialized, schema)
+    actual_hash_tree_root = SszEx.hash_tree_root!(real_deserialized, schema)
+
+    assert actual_hash_tree_root == expected_hash_tree_root
   end
 
   defp assert_ssz("invalid", schema, real_serialized) do
diff --git a/lib/ssz_ex.ex b/lib/ssz_ex.ex
index 536385ace..7ea566ce9 100644
--- a/lib/ssz_ex.ex
+++ b/lib/ssz_ex.ex
@@ -392,7 +392,8 @@ defmodule LambdaEthereumConsensus.SszEx do
   defp variable_size?({:bytes, _}), do: false
 
   defp pack(value, size) when is_integer(value) and value >= 0 do
-    <<value::size(size)-little>> |> String.pad_trailing(32, <<0>>)
+    pad = 256 - size
+    <<value::size(size)-little, 0::size(pad)>>
   end
 
   defp pack(value) when is_boolean(value) do