diff --git a/lib/glueby/contract/timestamp/tx_builder/simple.rb b/lib/glueby/contract/timestamp/tx_builder/simple.rb
index 5148dbe..70d6c4c 100644
--- a/lib/glueby/contract/timestamp/tx_builder/simple.rb
+++ b/lib/glueby/contract/timestamp/tx_builder/simple.rb
@@ -22,10 +22,7 @@ def set_data(prefix, data)
             @prefix = prefix
             @data = data
 
-            contents = [prefix, data].map do |content|
-              content.unpack1('H*')
-            end
-
+            contents = [prefix, data].map(&:bth)
             @txb.data(*contents)
             self
           end
diff --git a/spec/glueby/contract/timestamp/tx_builder/simple_spec.rb b/spec/glueby/contract/timestamp/tx_builder/simple_spec.rb
index 0014c5a..60d8861 100644
--- a/spec/glueby/contract/timestamp/tx_builder/simple_spec.rb
+++ b/spec/glueby/contract/timestamp/tx_builder/simple_spec.rb
@@ -94,7 +94,7 @@
     end
 
     context 'it gets a prefix that has a single hexadecimal digit' do
-      let(:prefix) { 'prefix
' }
+      let(:prefix) { "prefix\x01" }
 
       it 'set correct OP_RETURN data' do
         tx = builder.build
@@ -103,7 +103,7 @@
     end
 
     context 'it gets a content that has a single hexadecimal digit' do
-      let(:content) { 'content
' }
+      let(:content) { "content\x01" }
 
       it 'set correct OP_RETURN data' do
         tx = builder.build