Out of range problem in my linearsearch method

Question

So now I've been adding some code to my program.

Method LinearSearch:

static int LinearSearch(City[] cities, int key)
{   
    for (int i = 0; i < cities.Length; i++)
    {      
        if (cities[i].temp == key)
            return i;
    }
    return -1;  

} and then..

Console.Write("enter temperature to search for:");
string str = Console.ReadLine();
int key = Convert.ToInt32(str);

int index = LinearSearch(cities, key);


    if (index == -1)
        Console.WriteLine($"Couldn't find any city with temperature {key}");
    else   
        Console.WriteLine($"You searched for {key}");
        Console.WriteLine($"INDEX: {index}\t CITY: {cities[index].name} =\t TEMP: {cities[index].temp}");

When I then search for a value that's not in the array I first getting my answer: "Couldn't find any city with temperature {key}".

but then I get this messenge:

Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.