Replace First Occurrence of a String in C#

I had a need to replace only the first occurrence of a substring within a string, and C# doesn't seem to have a way of doing this with the out-of-the-box String methods. So, I rolled my own:

public static string ReplaceFirstOccurrence(string original, string oldValue, string newValue)
{
    if (String.IsNullOrEmpty(original))
        return String.Empty;
    if (String.IsNullOrEmpty(oldValue))
        return original;
    if (String.IsNullOrEmpty(newValue))
        newValue = String.Empty;
    int loc = original.IndexOf(oldValue);
    return original.Remove(loc, oldValue.Length).Insert(loc, newValue);
}

If you're interested, here is the test case for this method:

[TestClass]
    public class ReplaceFirstOccurrenceTest
    {
        [TestMethod]
        public void TestReplaceFirstOccurrence()
        {
            Assert.AreEqual("", StringUtil.ReplaceFirstOccurrence(null, null, null));
            Assert.AreEqual("", StringUtil.ReplaceFirstOccurrence("", "", ""));
            Assert.AreEqual("Th is what is right.",
                StringUtil.ReplaceFirstOccurrence("This is what is right.", "is", null));
            Assert.AreEqual("Th is what is right.",
                StringUtil.ReplaceFirstOccurrence("This is what is right.", "is", ""));
            Assert.AreEqual("This is what is right.",
                StringUtil.ReplaceFirstOccurrence("This is what is right.", null, ""));
            Assert.AreEqual("This is what is right.",
                StringUtil.ReplaceFirstOccurrence("This is what is right.", "", "stuff"));
        }
    }