diff --git a/pyrit/executor/attack/single_turn/many_shot_jailbreak.py b/pyrit/executor/attack/single_turn/many_shot_jailbreak.py
index 91ffcb6c44..d9b9d29f8d 100644
--- a/pyrit/executor/attack/single_turn/many_shot_jailbreak.py
+++ b/pyrit/executor/attack/single_turn/many_shot_jailbreak.py
@@ -65,12 +65,12 @@ def __init__(
             prompt_normalizer (PromptNormalizer, Optional): Normalizer for handling prompts.
             max_attempts_on_failure (int, Optional): Maximum number of attempts to retry on failure. Defaults to 0.
             example_count (int): The number of examples to include from many_shot_examples or the Many
-                Shot Jailbreaking dataset. Defaults to the first 100.
+                Shot Jailbreaking dataset. Must be a positive integer. Defaults to the first 100.
             many_shot_examples (list[dict[str, str]], Optional): The many shot jailbreaking examples to use.
                 If not provided, takes the first `example_count` examples from Many Shot Jailbreaking dataset.
 
         Raises:
-            ValueError: If many_shot_examples is empty.
+            ValueError: If example_count is not a positive integer or many_shot_examples is empty.
         """
         ManyShotJailbreakAttack.validate_constructor_parameters(
             {
@@ -106,12 +106,16 @@ def validate_constructor_parameters(cls, params: dict[str, Any]) -> None:
         Validate the supplied example selection without loading the default dataset.
 
         Raises:
-            ValueError: If the supplied example selection is empty.
+            ValueError: If ``example_count`` is not a positive integer or the supplied example
+                selection is empty.
         """
         super().validate_constructor_parameters(params)
         examples = params.get("many_shot_examples")
         count = params.get("example_count", 100)
-        if count == 0 or (examples is not None and not examples[:count]):
+        # The count is a slice bound, so a negative value would drop examples from the end.
+        if isinstance(count, bool) or not isinstance(count, int) or count < 1:
+            raise ValueError(f"example_count must be a positive integer, got {count!r}.")
+        if examples is not None and not examples:
             raise ValueError("Many shot examples must be provided.")
 
     async def _perform_async(self, *, context: SingleTurnAttackContext[Any]) -> AttackResult:
diff --git a/tests/unit/backend/test_technique_service.py b/tests/unit/backend/test_technique_service.py
index e9c6582fe4..ce3cf9c570 100644
--- a/tests/unit/backend/test_technique_service.py
+++ b/tests/unit/backend/test_technique_service.py
@@ -496,6 +496,7 @@ async def register_then_initialize_async(*, request: RunScenarioRequest, init_kw
         ("PromptSendingAttack", {"max_attempts_on_failure": -1}),
         ("ManyShotJailbreakAttack", {"max_attempts_on_failure": -1}),
         ("ManyShotJailbreakAttack", {"example_count": 0}),
+        ("ManyShotJailbreakAttack", {"example_count": -1}),
         ("ManyShotJailbreakAttack", {"many_shot_examples": []}),
         ("CrescendoAttack", {"max_backtracks": -1}),
         ("CrescendoAttack", {"max_turns": 0}),
diff --git a/tests/unit/executor/attack/single_turn/test_many_shot_jailbreak.py b/tests/unit/executor/attack/single_turn/test_many_shot_jailbreak.py
index f56fa46833..eee0962c22 100644
--- a/tests/unit/executor/attack/single_turn/test_many_shot_jailbreak.py
+++ b/tests/unit/executor/attack/single_turn/test_many_shot_jailbreak.py
@@ -188,6 +188,27 @@ def test_init_raises_error_with_empty_custom_examples(self, mock_from_yaml, mock
         with pytest.raises(ValueError, match="Many shot examples must be provided"):
             ManyShotJailbreakAttack(objective_target=mock_objective_target, many_shot_examples=[])
 
+    @pytest.mark.parametrize("example_count", [0, -1, -3, True, 2.5, "5", None])
+    @patch("pyrit.executor.attack.single_turn.many_shot_jailbreak.SeedPrompt.from_yaml_file")
+    def test_init_rejects_example_count_that_is_not_a_positive_integer(
+        self, mock_from_yaml, mock_objective_target, mock_template, sample_many_shot_examples, example_count
+    ):
+        """A negative count would slice examples off the end instead of selecting from the start."""
+        mock_from_yaml.return_value = mock_template
+
+        with pytest.raises(ValueError, match="example_count must be a positive integer"):
+            ManyShotJailbreakAttack(
+                objective_target=mock_objective_target,
+                example_count=example_count,
+                many_shot_examples=sample_many_shot_examples,
+            )
+
+    @pytest.mark.parametrize("example_count", [0, -1, True, "5"])
+    def test_validate_constructor_parameters_rejects_invalid_example_count(self, example_count):
+        """Deferred construction (e.g. the REST technique API) must reject the same counts."""
+        with pytest.raises(ValueError, match="example_count must be a positive integer"):
+            ManyShotJailbreakAttack.validate_constructor_parameters({"example_count": example_count})
+
 
 @pytest.mark.usefixtures("patch_central_database")
 class TestManyShotJailbreakAttackParamsType: