diff --git a/atomic-primops.cabal b/atomic-primops.cabal
--- a/atomic-primops.cabal
+++ b/atomic-primops.cabal
@@ -1,5 +1,5 @@
 Name:                atomic-primops
-Version:             0.2.2
+Version:             0.2.2.1
 License:             BSD3
 License-file:        LICENSE
 Author:              Ryan Newton
@@ -18,11 +18,16 @@
 -- 0.1.0.2 -- disable profiling
 -- 0.2 -- Critical bugfix and add Counter.
 -- 0.2.2 -- Add more counters
+-- 0.2.2.1 -- Minor, add warning.
 
 Synopsis: A safe approach to CAS and other atomic ops in Haskell.
 
 Description:
 
+  WARNING: Your program MUST be built with "ghc -threaded" if you depend on this library.
+           The library depends on threaded Haskell RTS, which is only conditionally linked.
+           If you don't do this you will see an error at link time about "cas" being missing.
+  . 
   After GHC 7.4 a new `casMutVar#` primop became available, but it's
   difficult to use safely, because pointer equality is a highly
   unstable property in Haskell.  This library provides a safer method