diff --git a/src/ppx/instrument.ml b/src/ppx/instrument.ml
index fe862384..f147da1c 100644
--- a/src/ppx/instrument.ml
+++ b/src/ppx/instrument.ml
@@ -1315,10 +1315,14 @@ class instrumenter =
 
           (* The case where we have [function A -> ... | B -> ...] *)
           | Pexp_function ([], constraint_, (Pfunction_cases _ as cases)) ->
-            traverse_function_body ~is_in_tail_position ~params:[] cases 
-            >>| fun (new_body, new_params) ->
-            let e = Ast_builder.Default.pexp_function ~loc new_params constraint_ new_body in
-            { e with pexp_attributes = attrs }
+            traverse_function_body ~is_in_tail_position cases
+            >>| fun new_body ->
+            (match (new_body : Parsetree.function_body) with
+            | Pfunction_body e ->
+              { e with pexp_attributes = attrs }
+            | Pfunction_cases _ ->
+              let e = Ast_builder.Default.pexp_function ~loc [] constraint_ new_body in
+              { e with pexp_attributes = attrs })
 
           (* Expressions that have subexpressions that might not get visited. *)
           | Pexp_function (params, constraint_, body) ->
@@ -1334,8 +1338,8 @@ class instrumenter =
             Ppxlib.With_errors.combine_errors new_params
             >>= fun new_params ->
 
-            traverse_function_body ~is_in_tail_position:true ~params:new_params body
-            >>| fun (new_body, new_params) ->
+            traverse_function_body ~is_in_tail_position:true body
+            >>| fun new_body ->
             let new_body =
               match new_body with
               | Pfunction_body { pexp_desc = Pexp_function _; _ } -> new_body
@@ -1655,12 +1659,12 @@ class instrumenter =
         end
         |> collect_errors
 
-      and traverse_function_body ~is_in_tail_position ~params body =
+      and traverse_function_body ~is_in_tail_position body =
         let open Ppxlib in
         match body with
         | Pfunction_body e ->
           traverse ~is_in_tail_position e
-          >>| fun e -> (Pfunction_body e, params)
+          >>| fun e -> Pfunction_body e
         | Pfunction_cases (cases, loc, attrs) ->
             traverse_cases ~is_in_tail_position:true cases
             >>| fun cases_new ->
@@ -1670,14 +1674,15 @@ class instrumenter =
                 Ast_builder.Default.pparam_val ~loc Nolabel None
                   [%pat? ___bisect_matched_value___]
               in
-              let body =
-                Pfunction_body
-                  (Exp.match_ ~loc
-                   ([%expr ___bisect_matched_value___]) cases)
-              in
-              (body, params @ [extra_param])
+              Pfunction_body
+                (Ast_builder.Default.pexp_function ~loc
+                   [extra_param]
+                   None
+                   (Pfunction_body
+                      (Exp.match_ ~loc
+                         ([%expr ___bisect_matched_value___]) cases)))
             else
-              (Pfunction_cases (cases, loc, attrs), params)
+              Pfunction_cases (cases, loc, attrs)
 
       in
 
diff --git a/test/instrument/function.t b/test/instrument/function.t
index 6441de3d..d8db1c5e 100644
--- a/test/instrument/function.t
+++ b/test/instrument/function.t
@@ -32,3 +32,25 @@ This is a regression test for #450: https://github.com/aantron/bisect_ppx/issues
 
   $ dune runtest --instrument-with bisect_ppx
   Test success!
+
+This is a regression test for https://github.com/aantron/bisect_ppx/pull/448#issuecomment-3477888423
+
+  $ cat > lib.ml << EOF
+  > let is_hex_digit (f : bool -> bool) : char -> bool = function
+  >   | '0' .. '9' | 'a' .. 'f' -> f true
+  >   | _ -> f false
+  > EOF
+
+  $ cat > test.ml <<'EOF'
+  > let () =
+  > if Lib.is_hex_digit (fun x -> x) '1' then begin
+  >   Printf.printf "Test success!";
+  >   exit 0
+  > end else
+  >   Printf.printf "Test failure!";
+  >   exit 1
+  > EOF
+
+  $ dune runtest --instrument-with bisect_ppx
+  Test success!
+