diff --git a/src/pipeline.ml b/src/pipeline.ml
index daaaa29..fa0de0f 100644
--- a/src/pipeline.ml
+++ b/src/pipeline.ml
@@ -174,13 +174,9 @@ let rec lower strict_unroll = function
   | Source.Case (s, (x, l), (y, r)) -> Target.Case (lower strict_unroll s, (x, lower strict_unroll l), (y, lower strict_unroll r))
 
 let synthesize_worker_wrapper ty body =
-  match ty with
-  | TArrow (TPair (TInt, TBool), TPair (TInt, TBool)) ->
-      let worker_body =
-        match body with
-        | Source.Lam (_, _, _, Source.Var _) -> Target.Tuple [Target.Var "u0"; Target.Var "u1"]
-        | _ -> lower false body
-      in
+  match ty, body with
+  | TArrow (TPair (TInt, TBool), TPair (TInt, TBool)), Source.Lam (arg, _, _, Source.Var result)
+    when String.equal arg result ->
       Target.WorkerWrapper
         {
           wrapper = "wrapper";
@@ -192,10 +188,12 @@ let synthesize_worker_wrapper ty body =
             Target.Let
               ("p", Target.Unbox (Target.Var "boxed"),
                Target.App (Target.Var "worker", [Target.Proj (0, Target.Var "p"); Target.Proj (1, Target.Var "p")]));
-          worker_body;
+          worker_body = Target.Tuple [Target.Var "u0"; Target.Var "u1"];
           in_term = Target.Var "wrapper";
         }
-  | _ -> lower false body
+  | TArrow (TPair (TInt, TBool), TPair (TInt, TBool)), _ ->
+      invalid_arg "unsupported worker-wrapper source shape"
+  | _ -> invalid_arg "unsupported worker-wrapper type"
 
 let compile flags source_type source_program =
   let obligations = ref [] in
diff --git a/test/invariants.ml b/test/invariants.ml
index e09c649..5c643b8 100644
--- a/test/invariants.ml
+++ b/test/invariants.ml
@@ -13,6 +13,13 @@ let assert_target_invalid msg term =
   | Ok () -> failwith msg
   | Error _ -> ()
 
+let assert_invalid_arg msg f =
+  try
+    let _ = f () in
+    failwith msg
+  with
+  | Invalid_argument _ -> ()
+
 let find_case name =
   match List.find_opt (fun (case : Corpus.case) -> String.equal case.name name) Corpus.all with
   | Some case -> case
@@ -72,4 +79,15 @@ let () =
          wrap_body = Target.Tuple [Target.Int 0; Target.Bool true];
          worker_body = Target.Tuple [Target.Var "u0"; Target.Bool true];
          in_term = Target.Var "wrapper";
-       })
+       });
+  assert_invalid_arg
+    "expected unsupported worker-wrapper source shape to be rejected"
+    (fun () ->
+       Pipeline.compile
+         { Pipeline.default_flags with unsafe_repr_eq = false; unsafe_strict_unroll = false }
+         (Types.TArrow (Types.TPair (Types.TInt, Types.TBool), Types.TPair (Types.TInt, Types.TBool)))
+         (Source.Lam
+            ( "p",
+              Types.TPair (Types.TInt, Types.TBool),
+              Types.TPair (Types.TInt, Types.TBool),
+              Source.Pair (Source.Int 0, Source.Bool true) )))