diff --git a/lib/jwe/enc/aes_cbc_hs.rb b/lib/jwe/enc/aes_cbc_hs.rb
index d48ccab..13b81af 100644
--- a/lib/jwe/enc/aes_cbc_hs.rb
+++ b/lib/jwe/enc/aes_cbc_hs.rb
@@ -64,11 +64,11 @@ def cek
       end
 
       def mac_key
-        cek[0...key_length / 2]
+        cek[0...(key_length / 2)]
       end
 
       def enc_key
-        cek[key_length / 2..-1]
+        cek[(key_length / 2)..-1]
       end
 
       def cipher