diff --git a/lib/mobi.rb b/lib/mobi.rb
index 3328c3b..95d6ae8 100644
--- a/lib/mobi.rb
+++ b/lib/mobi.rb
@@ -1,4 +1,5 @@
 require 'palm/pdb'
+require 'stringio'
 
 class Hash
   # 1.8 does not have Hash#key