C# function to return string error in unity, not all code paths return a value

I think there is something wrong with my for loop?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System;
using BreakInfinity;
using System.Numerics;

public class Notation : MonoBehaviour
{
    public int[] length;
    public string[] letter;

    void Start()
    {
        length = new int[11] { 7,10,13,16,19,21,24,27,30,33,36};
        letter = new string[11] { "K", "M", "B", "T", "q", "Q", "s", "S", "O", "N", "D" };
    }

    public BigDouble Divide(BigDouble num, int power)
    {
        return num / Math.Pow(10, power);
    }

    public string Notate(BigDouble num)
    {
        int digits = num.ToString().Length;

        if (num >= 10000)
        {
            if (digits > 36)
            {
                return "infinite";
            }
            else
            {
                for (int i = 0; i < 11; i++)
                {
                    return Divide(num, digits - 2).ToString("F2") + letter[i];
                }
            }
        }
        else
        {
            return num.ToString();
        }
    }
}