diff --git a/src/Lean/Compiler/LCNF/ReduceArity.lean b/src/Lean/Compiler/LCNF/ReduceArity.lean
index c3bd67db01..f5c4bc03fa 100644
--- a/src/Lean/Compiler/LCNF/ReduceArity.lean
+++ b/src/Lean/Compiler/LCNF/ReduceArity.lean
@@ -128,10 +128,12 @@ partial def reduce (code : Code) : ReduceM Code := do
   | .let decl k =>
     let .const declName _ args := decl.value | do return code.updateLet! decl (← reduce k)
     unless declName == (← read).declName do return code.updateLet! decl (← reduce k)
+    let mask := (← read).paramMask
     let mut argsNew := #[]
-    for used in (← read).paramMask, arg in args do
-      if used then
-        argsNew := argsNew.push arg
+    for h : i in *...args.size do
+      -- keep over-application
+      if mask.getD i true then
+        argsNew := argsNew.push args[i]
     let decl ← decl.updateValue (.const (← read).auxDeclName [] argsNew)
     return code.updateLet! decl (← reduce k)
   | .fun decl k | .jp decl k =>
diff --git a/tests/compiler/reduceArity_overapp.lean b/tests/compiler/reduceArity_overapp.lean
new file mode 100644
index 0000000000..2ece50f0a5
--- /dev/null
+++ b/tests/compiler/reduceArity_overapp.lean
@@ -0,0 +1,21 @@
+def tuple (types : List Type) : Type :=
+  match types with
+  | [] => Unit
+  | [t] => t
+  | t :: types => t × tuple types
+
+def uncurried ins out := tuple ins -> out
+
+def curried (ins : List Type) out := match ins with
+  | [] => out
+  | (x :: xs) => x -> curried xs out
+
+def curry (f : uncurried ins out) : curried ins out :=
+  match ins with
+  | [] => f ()
+  | [_] => f
+  | (_ :: _ :: _) => λx => curry (λxs => f (x, xs))
+
+def main : IO Unit := do
+  let val : String := curry (ins := [Int, String]) Prod.snd 1 "a"
+  IO.println val
diff --git a/tests/compiler/reduceArity_overapp.lean.expected.out b/tests/compiler/reduceArity_overapp.lean.expected.out
new file mode 100644
index 0000000000..533790e525
--- /dev/null
+++ b/tests/compiler/reduceArity_overapp.lean.expected.out
@@ -0,0 +1 @@
+a