diff --git a/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Util/Orphans.hs b/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Util/Orphans.hs
index 3a8795efaf..5d77ffab6b 100644
--- a/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Util/Orphans.hs
+++ b/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Util/Orphans.hs
@@ -22,6 +22,7 @@ import           Data.Bimap (Bimap)
 import qualified Data.Bimap as Bimap
 import           Data.IntPSQ (IntPSQ)
 import qualified Data.IntPSQ as PSQ
+import           Data.Monoid
 import           Data.SOP.BasicFunctors
 import           NoThunks.Class (InspectHeap (..), InspectHeapNamed (..),
                      NoThunks (..), OnlyCheckWhnfNamed (..), allNoThunks,
@@ -98,6 +99,8 @@ instance NoThunks a => NoThunks (K a b) where
   showTypeOf _ = showTypeOf (Proxy @a)
   wNoThunks ctxt (K a) = wNoThunks ("K":ctxt) a
 
+instance NoThunks a => NoThunks (Sum a)
+
 {-------------------------------------------------------------------------------
   fs-api
 -------------------------------------------------------------------------------}