Why does a space preceding a non-combining diacritic function differently when using IndexOf(string) and IndexOf(char)?

I am creating a substring from a string with non-combining diacritics that follow a space. When doing so, I check the string with .Contains() and then perform the substring. When I use a space char inside of an .IndexOf(), the program performs as expected, yet when using the string " ", within .IndexOf() the program throws an exception. As shown in the samples below only a string preceding the primary stress diacritic (U+02C8) throws an ArgumentOutOfRangeException.

Simple code (Edit suggested by John):

string a = "aɪ prɪˈzɛnt";
string b = "maɪ ˈprɛznt";

// A            
Console.WriteLine(a.IndexOf(" ")); // string index:  2
Console.WriteLine(a.IndexOf(' ')); // char index:    2

// B    
Console.WriteLine(b.IndexOf(" ")); // string index: -1
Console.WriteLine(b.IndexOf(' ')); // char index:    3

Sample code I tested with:

        const string iPresent = "aɪ prɪˈzɛnt",
                     myPresent = "maɪ ˈprɛznt";

        if(iPresent.Contains(' '))
        {
            Console.WriteLine(iPresent.Substring(0, iPresent.IndexOf(' ')));
        }

        if(iPresent.Contains(" "[0]))
        {
            Console.WriteLine(iPresent.Substring(0, iPresent.IndexOf(" "[0])));
        }

        if(iPresent.Contains(" "))
        {
            Console.WriteLine(iPresent.Substring(0, iPresent.IndexOf(" ")));
        }

        if(iPresent.Contains(string.Empty + ' '))
        {
            Console.WriteLine(iPresent.Substring(0, iPresent.IndexOf(string.Empty + ' ')));
        }

        if (myPresent.Contains(' '))
        {
            Console.WriteLine(myPresent.Substring(0, myPresent.IndexOf(' ')));
        }

        if (myPresent.Contains(" "[0]))
        {
            Console.WriteLine(myPresent.Substring(0, myPresent.IndexOf(" "[0])));
        }

        if (myPresent.Contains(string.Empty + ' '))
        {
            try
            {
                Console.WriteLine(myPresent.Substring(0, myPresent.IndexOf(string.Empty + ' ')));
            }
            catch (Exception ex)
            {
                Console.WriteLine("***" + ex.Message);
            }
        }

        if (myPresent.Contains(" "))
        {
            try
            {
                Console.WriteLine(myPresent.Substring(0, myPresent.IndexOf(" ")));
            }
            catch (Exception ex)
            {
                Console.WriteLine("***" + ex.Message);
            }
        }