diff --git a/pyrit/converter/add_image_text_converter.py b/pyrit/converter/add_image_text_converter.py
index 87715bd9d0..0906c91daa 100644
--- a/pyrit/converter/add_image_text_converter.py
+++ b/pyrit/converter/add_image_text_converter.py
@@ -65,7 +65,7 @@ def __init__(
 
         Raises:
             ValueError: If img_to_add is empty, font_name doesn't end with ".ttf",
-                font_size tuple is invalid, or bounding_box coordinates are invalid.
+                font_size is invalid, or bounding_box coordinates are invalid.
         """
         if not img_to_add:
             raise ValueError("Please provide valid image path")
@@ -119,7 +119,7 @@ def _extract_font_size(self, font_size: int | tuple[int, int]) -> None:
             font_size (int | tuple[int, int]): Fixed size or (min, max) range.
 
         Raises:
-            ValueError: If font_size tuple is invalid.
+            ValueError: If font_size is not positive or the tuple range is invalid.
         """
         if isinstance(font_size, tuple):
             if len(font_size) != 2 or font_size[0] > font_size[1] or font_size[0] < 1:
@@ -128,6 +128,8 @@ def _extract_font_size(self, font_size: int | tuple[int, int]) -> None:
             self._font_size_max = font_size[1]
             self._auto_font_size = True
         else:
+            if font_size < 1:
+                raise ValueError("font_size must be greater than 0")
             self._font_size_min = font_size
             self._font_size_max = font_size
             self._auto_font_size = False
diff --git a/tests/unit/converter/test_add_image_text_font_size_validation.py b/tests/unit/converter/test_add_image_text_font_size_validation.py
new file mode 100644
index 0000000000..c69ddc28c0
--- /dev/null
+++ b/tests/unit/converter/test_add_image_text_font_size_validation.py
@@ -0,0 +1,16 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT license.
+
+import pytest
+from PIL import Image
+
+from pyrit.converter import AddImageTextConverter
+
+
+@pytest.mark.parametrize("font_size", [0, -1])
+def test_add_image_text_converter_rejects_non_positive_fixed_font_size(tmp_path, font_size):
+    image_path = tmp_path / "test.png"
+    Image.new("RGB", (32, 32)).save(image_path)
+
+    with pytest.raises(ValueError, match="font_size must be greater than 0"):
+        AddImageTextConverter(img_to_add=str(image_path), font_size=font_size)