diff --git a/src/Core2Core.hs b/src/Core2Core.hs
index a187a63..d1bcfe3 100644
--- a/src/Core2Core.hs
+++ b/src/Core2Core.hs
@@ -70,7 +70,7 @@ tagData p = let ?dt = p ^. programDataTags
     go x                = embed x
 
     tagAlts :: (?dt :: HashMap Name (Tag, Int)) => Alter' -> Alter'
-    tagAlts (Alter (AltData c) bs e) = Alter (AltTag tag) bs e
+    tagAlts (Alter (AltData c) bs e) = Alter (AltTag tag) bs (cata go e)
         where tag = case ?dt ^. at c of
                 Just (t,_) -> t
                 -- TODO: errorful