diff --git a/randcool/randcool.hs b/randcool/randcool.hs
index 96839b9a7dab723a26770902691f8344111ec993..cfddcafafe64c73c9b2ba45da35d65b032f0f398 100644
--- a/randcool/randcool.hs
+++ b/randcool/randcool.hs
@@ -107,7 +107,7 @@ numAgents n = CL <$> buildSet I.empty (n+1)
     buildSet !acc m
       | m <= 0    = pure acc
       | otherwise = do heads <- rand
-                       let acc' = if heads then acc else I.insert (m-1) acc
+                       let acc' = if heads then acc else I.insert m acc
                        buildSet acc' (m-1)
 
 someFormula :: Rand mod -> RandConf -> Rand (Fm mod)