diff --git a/src/ytdl_sub/script/functions/string_functions.py b/src/ytdl_sub/script/functions/string_functions.py
index 574b6113..d54f0744 100644
--- a/src/ytdl_sub/script/functions/string_functions.py
+++ b/src/ytdl_sub/script/functions/string_functions.py
@@ -55,8 +55,8 @@ class StringFunctions:
         return String(string.value.replace(old.value, new.value))
 
     @staticmethod
-    def concat(*args: String) -> String:
+    def concat(*values: String) -> String:
         """
         Concatenate multiple Strings into a single String.
         """
-        return String("".join(*args))
+        return String("".join(list([val.value for val in values])))
diff --git a/tests/unit/script/functions/test_string_functions.py b/tests/unit/script/functions/test_string_functions.py
new file mode 100644
index 00000000..7352debe
--- /dev/null
+++ b/tests/unit/script/functions/test_string_functions.py
@@ -0,0 +1,78 @@
+import pytest
+from unit.script.conftest import single_variable_output
+
+from ytdl_sub.script.script import Script
+
+
+class TestNumericFunctions:
+    @pytest.mark.parametrize(
+        "values, expected_output",
+        [
+            ("'lower'", "LOWER"),
+            ("'UPPER'", "UPPER"),
+        ],
+    )
+    def test_upper(self, values: str, expected_output: str):
+        output = single_variable_output(f"{{%upper({values})}}")
+        assert output == expected_output
+
+    @pytest.mark.parametrize(
+        "values, expected_output",
+        [
+            ("'lower'", "lower"),
+            ("'UPPER'", "upper"),
+        ],
+    )
+    def test_lower(self, values: str, expected_output: str):
+        output = single_variable_output(f"{{%lower({values})}}")
+        assert output == expected_output
+
+    @pytest.mark.parametrize(
+        "values, expected_output",
+        [
+            ("'lower First word'", "Lower first word"),
+            ("'UPPER First word'", "Upper first word"),
+        ],
+    )
+    def test_capitalize(self, values: str, expected_output: str):
+        output = single_variable_output(f"{{%capitalize({values})}}")
+        assert output == expected_output
+
+    @pytest.mark.parametrize(
+        "values, expected_output",
+        [
+            ("'lower First word'", "Lower First Word"),
+            ("'UPPER First word'", "Upper First Word"),
+        ],
+    )
+    def test_titlecase(self, values: str, expected_output: str):
+        output = single_variable_output(f"{{%titlecase({values})}}")
+        assert output == expected_output
+
+    @pytest.mark.parametrize(
+        "values, expected_output",
+        [
+            (
+                "'lower First word second word', 'word', 'string'",
+                "lower First string second string",
+            ),
+            (
+                "'lower First word second word', 'word', 'string', 1",
+                "lower First string second word",
+            ),
+        ],
+    )
+    def test_replace(self, values: str, expected_output: str):
+        output = single_variable_output(f"{{%replace({values})}}")
+        assert output == expected_output
+
+    @pytest.mark.parametrize(
+        "values, expected_output",
+        [
+            ("'lower First word'", "lower First word"),
+            ("'lower First word', ' second', ' string'", "lower First word second string"),
+        ],
+    )
+    def test_concat(self, values: str, expected_output: str):
+        output = single_variable_output(f"{{%concat({values})}}")
+        assert output == expected_output