Function will not return anything

Unit testing a function but the function isn't returning anything when I run the unit test.

This is the function

    public string FindUniqueWords(Dictionary<string, int> dictionary)
    {
        string uniqueWord = "";
        foreach (KeyValuePair<string, int> pair in dictionary) //loop through the occurenceDictionary
        {
            uniqueWord += (pair.Key + ", ");
        }
        uniqueWords.Text = uniqueWord;
        return (uniqueWord);
    }

And this is the unit test

    [TestMethod]
    public void CheckUniqueWordsAreFoundTest()
    {
        string actual, expected;
        Form1 frm1 = new Form1();
        actual = frm1.FindUniqueWords(unitTestOccurenceDictionary);
        expected = "hello, world, i, am, god, of, this";
        Assert.AreEqual(expected, actual);
    }

And I am sending this dictionary to the function

public class UnitTest1
{
    public Dictionary<string, int> unitTestOccurenceDictionary = new Dictionary<string, int>();
    public void Add(string key, string value)
    {
        unitTestOccurenceDictionary.Add("hello", 1);
        unitTestOccurenceDictionary.Add("world", 2);
        unitTestOccurenceDictionary.Add("I", 1);
        unitTestOccurenceDictionary.Add("am", 1);
        unitTestOccurenceDictionary.Add("god", 1);
        unitTestOccurenceDictionary.Add("of", 1);
        unitTestOccurenceDictionary.Add("this", 1);
    }

When I run the unit test it fails and shows that actual = <> I am not sure what is causing this but I feel like it's a stupidly simple mistake like a missing reference but there are no errors when I run my code.