diff --git a/lib/Quox/Parser/Parser.idr b/lib/Quox/Parser/Parser.idr
index f8b9496..08c27ef 100644
--- a/lib/Quox/Parser/Parser.idr
+++ b/lib/Quox/Parser/Parser.idr
@@ -166,7 +166,9 @@ lamIntro = symbolsC [(Lam, "λ"), (DLam, "δ")]
 private covering
 caseIntro : Grammar True PQty
 caseIntro = symbols [(One, "case1"), (Any, "caseω")]
-        <|> resC "case" *> qty <* resC "."
+        <|> resC "case" *>
+              (qty <* resC "." <|>
+               fatalError {c = True} "missing quantity on 'case'")
 
 private
 optNameBinder : Grammar False BName