diff --git a/README.rdoc b/README.rdoc
index c16ba428..11448e6f 100644
--- a/README.rdoc
+++ b/README.rdoc
@@ -12,4 +12,5 @@ If you want to build the gem from source:
 
 == REQUIREMENTS:
 
+* Ruby 1.8.7 or above. (Ruby 1.8.6 may work if you load ActiveSupport.)
 * rest-client, json