diff --git a/lib/Quox/Typechecker.idr b/lib/Quox/Typechecker.idr
index f5d6235..2be7811 100644
--- a/lib/Quox/Typechecker.idr
+++ b/lib/Quox/Typechecker.idr
@@ -320,12 +320,15 @@ parameters {auto _ : IsQty q} {auto _ : CanTC q m}
     expectCompatQ one pi
     -- if Ψ | Γ ⊢ σ · t ⇒ {ts} ⊳ Σ₁
     tres <- inferC ctx sg t
+    ttags <- expectEnum !ask ctx tres.type
     -- if Ψ | Γ, x : {ts} ⊢₀ A ⇐ Type
     checkTypeC (extendTy zero ret.name tres.type ctx) ret.term Nothing
     -- if for each "a ⇒ s" in arms,
     --   Ψ | Γ ⊢ σ · s ⇐ A[a ∷ {ts}/x] ⊳ Σ₂
     -- for fixed Σ₂
     let arms = SortedMap.toList arms
+    let armTags = SortedSet.fromList $ map fst arms
+    unless (ttags == armTags) $ throwError $ BadCaseEnum ttags armTags
     armres <- for arms $ \(a, s) =>
       checkC ctx sg s (sub1 ret (Tag a :# tres.type))
     armout <- allEqual (zip armres arms)
diff --git a/lib/Quox/Typing/Error.idr b/lib/Quox/Typing/Error.idr
index 930ab8d..a6ae092 100644
--- a/lib/Quox/Typing/Error.idr
+++ b/lib/Quox/Typing/Error.idr
@@ -20,6 +20,7 @@ data Error q
 | ExpectedNat  (TyContext q d n) (Term q d n)
 | BadUniverse  Universe Universe
 | TagNotIn     TagVal (SortedSet TagVal)
+| BadCaseEnum  (SortedSet TagVal) (SortedSet TagVal)
 | BadCaseQtys  (TyContext q d n) (List (QOutput q n, Term q d n, Term q d n))
 
 -- first term arg of ClashT is the type
@@ -218,6 +219,10 @@ parameters {auto _ : (Eq q, IsQty q, PrettyHL q)} (unicode : Bool)
       sep [sep ["tag", prettyTag tag, "is not contained in"],
            termt empty (Enum set)]
 
+    BadCaseEnum type arms =>
+      sep ["case expression has head of type", termt empty (Enum type),
+           "but cases for", termt empty (Enum arms)]
+
     BadCaseQtys ctx arms =>
       hang 4 $ sep $
         "inconsistent variable usage in case arms" ::