diff --git a/pyrit/converter/unicode_sub_converter.py b/pyrit/converter/unicode_sub_converter.py
index 79ce3eef22..f16a1ed8c0 100644
--- a/pyrit/converter/unicode_sub_converter.py
+++ b/pyrit/converter/unicode_sub_converter.py
@@ -20,7 +20,12 @@ def __init__(self, *, start_value: int = 0xE0000) -> None:
 
         Args:
             start_value (int): The unicode starting point to use for encoding.
+
+        Raises:
+            ValueError: If ``start_value`` is outside the Unicode code point range.
         """
+        if not 0 <= start_value <= 0x10FFFF:
+            raise ValueError("start_value must be a valid Unicode code point between 0 and 0x10FFFF")
         self.startValue = start_value
 
     def _build_identifier(self) -> ComponentIdentifier:
diff --git a/tests/unit/converter/test_unicode_sub_converter.py b/tests/unit/converter/test_unicode_sub_converter.py
index f76a2df660..3084550fc6 100644
--- a/tests/unit/converter/test_unicode_sub_converter.py
+++ b/tests/unit/converter/test_unicode_sub_converter.py
@@ -22,6 +22,12 @@ async def test_unicode_sub_custom_start():
     assert result.output_type == "text"
 
 
+@pytest.mark.parametrize("start_value", [-1, 0x110000])
+def test_unicode_sub_rejects_invalid_start_value(start_value):
+    with pytest.raises(ValueError, match="valid Unicode code point"):
+        UnicodeSubstitutionConverter(start_value=start_value)
+
+
 async def test_unicode_sub_empty():
     converter = UnicodeSubstitutionConverter()
     result = await converter.convert_async(prompt="", input_type="text")