Syntax error regarding line 15 characters 27-34 in an OCaml tree search function

I wrote the function below to look through a tree and return a list in increasing order that contains all the indexes/subscripts of even elements in the tree. However I am getting a syntax error at the line let rec GetSubEvenGen tree counter list = about the argument counter. When prompting chat GPT about this it failed to spot any errors. I'd appreciate any advice on why a syntax error is being raised.

 type 'a tree =   |Lf   |Br of 'a * 'a tree * 'a tree;; exception Subscriptlet rec look = function  | Lf, _ -> raise Subscript   | Br (v, t1, t2), k ->      if k = 1 then v      else if k mod 2 = 0 then        look (t1, k / 2)      else        look (t2, k / 2);; let rec GetSubEvenGen tree counter list =   try    let element = look (tree, counter) in    if element mod 2 = 0 then      GetSubEvenGen tree (counter + 1) (element::list)    else      GetSubEvenGen tree (counter + 1) (list)  with     Subscript -> List.rev(list);;let getSubsOfEvens tree = GetSubEvenGen tree 1 [];;

I tried running this code, I expected it to be syntactically correct, however a syntax error was raised.