diff --git a/packages/belt/src/belt_Array.ml b/packages/belt/src/belt_Array.ml
index 1c3b74c1b..5545195f2 100644
--- a/packages/belt/src/belt_Array.ml
+++ b/packages/belt/src/belt_Array.ml
@@ -503,19 +503,18 @@ let reduceWithIndexU a x f =
 
 let reduceWithIndex a x f = reduceWithIndexU a x (fun a b c -> f a b c)
 
-let joinWithU a sep toString =
+let joinWithU a sep =
   match length a with
   | 0 -> ""
   | l ->
       let lastIndex = l - 1 in
       let rec aux i res =
         let v = getUnsafe a i in
-        if i = lastIndex then res ^ toString v
-        else aux (i + 1) (res ^ toString v ^ sep)
+        if i = lastIndex then res ^ v else aux (i + 1) (res ^ v ^ sep)
       in
       aux 0 ""
 
-let joinWith a sep toString = joinWithU a sep (fun x -> toString x)
+let joinWith a sep = joinWithU a sep
 let initU n f = Stdlib.Array.init n f
 let init n f = initU n (fun i -> f i)
 
diff --git a/packages/belt/src/belt_Array.mli b/packages/belt/src/belt_Array.mli
index ca4ec0e54..c50a57c94 100644
--- a/packages/belt/src/belt_Array.mli
+++ b/packages/belt/src/belt_Array.mli
@@ -549,9 +549,9 @@ val reduceWithIndex : 'a t -> 'b -> ('b -> 'a -> int -> 'b) -> 'b
     ]}
 *)
 
-val joinWithU : 'a t -> string -> (('a -> string)[@bs]) -> string
+val joinWithU : string t -> string -> string
 
-val joinWith : 'a t -> string -> ('a -> string) -> string
+val joinWith : string t -> string -> string
 (** [joinWith xs sep toString]
 
     Concatenates all the elements of [xs] converted to string with [toString], each separated by [sep], the string