❔ System.ArgumentOutOfRangeException: Index and length must refer to a location within the string.

using System;
using System.IO;
using System.Collections.Generic;
internal class Program
{
    public static void Main(string[] args)
    {
        // Create a string array for all items due to be compiled
        List<string> ghouLines = new List<string>();
        string buildDirectory = Directory.GetCurrentDirectory(); // Build directory
        while (true) {
            string input = Console.ReadLine();
            // Variables
            if (input.Substring(0,3) == "mut") {
                int equals = input.IndexOf('=') - 1;
                string variableName = input.Substring(3,equals);
                int endArg = input.IndexOf(';');
                string variableData = input.Substring(equals,endArg);
                ghouLines.Add($"var {variableName} {variableData};");
            }
            ghouLines.ForEach(i => Console.Write("{0}\t", i)); // DEBUG
        }
    }
}

mut a = 3;