Issue with Random string generator producing identical results

I use a random string generator, based on this: http://ift.tt/2xIElkV

Every now and then, it will generate a 20 chars len string like this aaaaaaaaaaaaaaaaaaaa when it needs to generate a 20 chars len string full of random chars (eg 63TSRVvbVDJiMNwneB5l), like if the C# Random object would return a value of 0 every time over the 20 iterations.

public static string GetRandomAlphaNumericString(int charCount)
{
  var result = new string(
      Enumerable.Repeat(CHARS, charCount)
                .Select(s =>
                {
                  var nxt = SHARED_RANDOM.Next(s.Length);
                  var rval = s[nxt];

                  return rval;
                })
                .ToArray());
  return result;
}

const string CHARS = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

public static Random SHARED_RANDOM = new Random(Guid.NewGuid().GetHashCode());

It doesn't make sense to me, what could I be overseeing in this generator ? The generator works OK, but every now and then, very rarely, it would perform like this generating a strike aaaaaaa strings for a short period of time.

I don't see a bug in the code, so to me it's 1 of 2 things, or the C# Random object is acting funny in some situations, returning 0 for a short period of time, or the CHARS const string changes from "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789" to "a" in the assembly for some reason, for a short period of time.

I can't believe it could be either of the above, I prefer to believe that I'm overseeing something. Do you see or imagine how could this possibly be happening ?