diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index a425bc3b9ad1b6007199a527764912b74de7f12b..2433eb8ba7cb1229208d0b68533ce0145dd719ac 100644
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -259,6 +259,7 @@ mkAltExpr (DataAlt con) args inst_tys
   = mkConApp con (map Type inst_tys ++ varsToCoreExprs args)
 mkAltExpr (LitAlt lit) [] []
   = Lit lit
+mkAltExpr DEFAULT = panic "mkAltExpr"
 
 mkIfThenElse :: CoreExpr -> CoreExpr -> CoreExpr -> CoreExpr
 mkIfThenElse guard then_expr else_expr