diff --git a/lib/miscreant/aes/siv.rb b/lib/miscreant/aes/siv.rb
index 3cf2cdd..d1f8eb6 100644
--- a/lib/miscreant/aes/siv.rb
+++ b/lib/miscreant/aes/siv.rb
@@ -68,7 +68,7 @@ def seal(plaintext, associated_data = [])
       # @raise [Miscreant::IntegrityError] ciphertext and/or associated data are corrupt or tampered with
       # @return [String] decrypted plaintext
       def open(ciphertext, associated_data = [])
-        raise TypeError, "expected String, got #{ciphertext.class}" unless ciphertext.is_a?(String)
+        Internals::Util.validate_bytestring("ciphertext", ciphertext)
 
         v = ciphertext[0, Internals::Block::SIZE]
         plaintext = @ctr.encrypt(_zero_iv_bits(v), ciphertext[Internals::Block::SIZE..-1])
diff --git a/spec/miscreant/aead_spec.rb b/spec/miscreant/aead_spec.rb
index de71534..fbd9207 100644
--- a/spec/miscreant/aead_spec.rb
+++ b/spec/miscreant/aead_spec.rb
@@ -53,6 +53,15 @@
           end.to raise_error(Miscreant::IntegrityError)
         end
       end
+
+      it "should raise ArgumentError if wrong encoding" do
+        ex = test_vectors.first
+        aead = described_class.new(ex.alg, ex.key)
+        ciphertext = ex.ciphertext.dup.force_encoding(Encoding::UTF_8)
+        expect do
+          aead.open(ciphertext, nonce: example_nonce, ad: example_ad)
+        end.to raise_error(ArgumentError, "ciphertext must be Encoding::BINARY")
+      end
     end
   end
 end