diff --git a/feedback.yaml b/feedback.yaml
index 87f6e516..547c4195 100644
--- a/feedback.yaml
+++ b/feedback.yaml
@@ -1,4 +1,4 @@
 loops:
-  build: cabal build -j -O0 all
+  build: cabal build -j -O0 --ghc-options='-Wall -Werror' all
   test: cabal test -j -O0 --test-show-details=direct swarm:swarm-integration swarm:swarm-unit
   unit: cabal test -j -O0 --test-show-details=direct swarm:swarm-unit
diff --git a/src/swarm-engine/Swarm/Game/Step.hs b/src/swarm-engine/Swarm/Game/Step.hs
index 8e137392..45611f05 100644
--- a/src/swarm-engine/Swarm/Game/Step.hs
+++ b/src/swarm-engine/Swarm/Game/Step.hs
@@ -847,11 +847,9 @@ stepCESK cesk = case cesk of
     let s' = resetBlackholes s
     h <- hasCapability CLog
     em <- use $ landscape . terrainAndEntities . entityMap
-    if h
-      then do
-        void $ traceLog RobotError Error (formatExn em exn)
-        return $ Out VUnit s []
-      else return $ Out VUnit s' []
+    when h $ void $ traceLog RobotError Error (formatExn em exn)
+    return $ Out VExc s' []
+
   -- Fatal errors, capability errors, and infinite loop errors can't
   -- be caught; just throw away the continuation stack.
   Up exn@Fatal {} s _ -> return $ Up exn s []
diff --git a/src/swarm-engine/Swarm/Game/Step/Arithmetic.hs b/src/swarm-engine/Swarm/Game/Step/Arithmetic.hs
index 18c06ee5..b07cc394 100644
--- a/src/swarm-engine/Swarm/Game/Step/Arithmetic.hs
+++ b/src/swarm-engine/Swarm/Game/Step/Arithmetic.hs
@@ -75,6 +75,7 @@ compareValues v1 = case v1 of
   VDelay {} -> incomparable v1
   VRef {} -> incomparable v1
   VRequirements {} -> incomparable v1
+  VExc {} -> incomparable v1
 
 -- | Values with different types were compared; this should not be
 --   possible since the type system should catch it.
diff --git a/src/swarm-lang/Swarm/Language/Value.hs b/src/swarm-lang/Swarm/Language/Value.hs
index 5871dc6a..c618506d 100644
--- a/src/swarm-lang/Swarm/Language/Value.hs
+++ b/src/swarm-lang/Swarm/Language/Value.hs
@@ -92,6 +92,9 @@ data Value where
   VKey :: KeyCombo -> Value
   -- | A 'requirements' command awaiting execution.
   VRequirements :: Text -> Term -> Env -> Value
+  -- | A special value representing a program that terminated with
+  --   an exception.
+  VExc :: Value
   deriving (Eq, Show, Generic)
 
 instance ToJSON Value where
@@ -133,6 +136,7 @@ valueToTerm (VRef n) = TRef n
 valueToTerm (VRcd m) = TRcd (Just . valueToTerm <$> m)
 valueToTerm (VKey kc) = TApp (TConst Key) (TText (prettyKeyCombo kc))
 valueToTerm (VRequirements x t _) = TRequirements x t
+valueToTerm VExc = TConst Undefined
 
 -- | An environment is a mapping from variable names to values.
 type Env = Ctx Value
diff --git a/src/swarm-tui/Swarm/TUI/Controller.hs b/src/swarm-tui/Swarm/TUI/Controller.hs
index 600a3691..379fbc66 100644
--- a/src/swarm-tui/Swarm/TUI/Controller.hs
+++ b/src/swarm-tui/Swarm/TUI/Controller.hs
@@ -104,7 +104,7 @@ import Swarm.Language.Requirement qualified as R
 import Swarm.Language.Syntax hiding (Key)
 import Swarm.Language.Typed (Typed (..))
 import Swarm.Language.Types
-import Swarm.Language.Value (Value (VKey, VUnit), prettyValue, stripVResult)
+import Swarm.Language.Value (Value (VExc, VKey, VUnit), prettyValue, stripVResult)
 import Swarm.Log
 import Swarm.TUI.Controller.Util
 import Swarm.TUI.Editor.Controller qualified as EC
@@ -829,26 +829,28 @@ updateUI = do
 
   -- Now check if the base finished running a program entered at the REPL.
   replUpdated <- case g ^. gameControls . replStatus of
-    -- It did, and the result was the unit value.  Just reset replStatus.
-    REPLWorking (Typed (Just VUnit) typ reqs) -> do
-      gameState . gameControls . replStatus .= REPLDone (Just $ Typed VUnit typ reqs)
-      pure True
+    REPLWorking (Typed (Just v) pty reqs)
+      -- It did, and the result was the unit value or an exception.  Just reset replStatus.
+      | v `elem` [VUnit, VExc] -> do
+          gameState . gameControls . replStatus .= REPLDone (Just $ Typed v pty reqs)
+          pure True
 
-    -- It did, and returned some other value.  Pretty-print the
-    -- result as a REPL output, with its type, and reset the replStatus.
-    REPLWorking (Typed (Just v) pty reqs) -> do
-      let finalType = stripCmd pty
-      let val = Typed (stripVResult v) finalType reqs
-      itIx <- use (gameState . gameControls . replNextValueIndex)
-      let itName = fromString $ "it" ++ show itIx
-      let out = T.intercalate " " [itName, ":", prettyText finalType, "=", into (prettyValue v)]
-      uiState . uiGameplay . uiREPL . replHistory %= addREPLItem (REPLOutput out)
-      invalidateCacheEntry REPLHistoryCache
-      vScrollToEnd replScroll
-      gameState . gameControls . replStatus .= REPLDone (Just val)
-      gameState . baseRobot . robotContext . at itName .= Just val
-      gameState . gameControls . replNextValueIndex %= (+ 1)
-      pure True
+      -- It did, and returned some other value.  Create a new 'it'
+      -- variable, pretty-print the result as a REPL output, with its
+      -- type, and reset the replStatus.
+      | otherwise -> do
+          itIx <- use (gameState . gameControls . replNextValueIndex)
+          let finalType = stripCmd pty
+              val = Typed (stripVResult v) finalType reqs
+              itName = fromString $ "it" ++ show itIx
+              out = T.intercalate " " [itName, ":", prettyText finalType, "=", into (prettyValue v)]
+          uiState . uiGameplay . uiREPL . replHistory %= addREPLItem (REPLOutput out)
+          invalidateCacheEntry REPLHistoryCache
+          vScrollToEnd replScroll
+          gameState . gameControls . replStatus .= REPLDone (Just val)
+          gameState . baseRobot . robotContext . at itName .= Just val
+          gameState . gameControls . replNextValueIndex %= (+ 1)
+          pure True
 
     -- Otherwise, do nothing.
     _ -> pure False
diff --git a/src/swarm-tui/Swarm/TUI/Model.hs b/src/swarm-tui/Swarm/TUI/Model.hs
index 4a750340..df5b3d83 100644
--- a/src/swarm-tui/Swarm/TUI/Model.hs
+++ b/src/swarm-tui/Swarm/TUI/Model.hs
@@ -93,6 +93,8 @@ import Swarm.Game.State.Runtime
 import Swarm.Game.State.Substate
 import Swarm.Game.Tick (TickNumber (..))
 import Swarm.Game.World.Gen (Seed)
+import Swarm.Language.Typed (Typed (..))
+import Swarm.Language.Value (Value (VExc))
 import Swarm.Log
 import Swarm.TUI.Inventory.Sorting
 import Swarm.TUI.Model.Menu
@@ -317,5 +319,5 @@ topContext s = ctxPossiblyWithIt
   ctx = fromMaybe emptyRobotContext $ s ^? gameState . baseRobot . robotContext
 
   ctxPossiblyWithIt = case s ^. gameState . gameControls . replStatus of
-    REPLDone (Just p) -> ctx & at "it" ?~ p
+    REPLDone (Just p@(Typed v _ _)) | v /= VExc -> ctx & at "it" ?~ p
     _ -> ctx