Android Kotlin Firebase "Error Invalid document reference. Document references must have an even number of segments, but products has 1"

using kotlin

fun upadteProductInterestedList(activity: Activity, productId: String, uid: String) {
            mFirestore.collection(Constants.PRODUCTS)
                .document(productId)
                .update(Constants.PRODUCT_INTERESTED, FieldValue.arrayUnion(uid))
                .addOnSuccessListener {e ->
                    when(activity) {
                        is CheckProductDetailsActivity -> {
                            activity.productInterestedSuccess()
                        }
                    }
                }
                .addOnFailureListener { e ->
                    when(activity) {
                        is CheckProductDetailsActivity -> {
                            activity.hideProgressDialog()
                        }
                    }
                    Log.e(
                        activity.javaClass.simpleName,
                        "Error while updating profile",
                        e
                    )
                }
    }

I am using this function to update a array of ids inside firebase document and getting

"java.lang.IllegalArgumentException: Invalid document reference. Document references must have an even number of segments, but products has 1" error when the function is called. Error (E/AndroidRuntime: FATAL EXCEPTION: main) also mentions where this function was called.

The documents in product collection have a field called "interested" which is a array of strings. i am trying to store the user id when user clickes on the button.