diff --git a/tests/test_costs.py b/tests/test_costs.py
index 96f5b86c..724214d0 100644
--- a/tests/test_costs.py
+++ b/tests/test_costs.py
@@ -271,3 +271,8 @@ def test_calculate_cached_tokens_cost():
     # Assert that the costs match
     assert actual_cost == expected_cost
     assert actual_cost > 0, "Cache token cost should be greater than zero"
+
+
+def test_calculate_cost_by_tokens_rejects_negative_token_counts():
+    with pytest.raises(ValueError, match="num_tokens must be non-negative"):
+        calculate_cost_by_tokens(-1, "gpt-4o", "input")
diff --git a/tokencost/costs.py b/tokencost/costs.py
index cc4680ff..7e276372 100644
--- a/tokencost/costs.py
+++ b/tokencost/costs.py
@@ -317,6 +317,9 @@ def calculate_cost_by_tokens(num_tokens: int, model: str, token_type: TokenType)
     Returns:
         Decimal: The calculated cost in USD.
     """
+    if num_tokens < 0:
+        raise ValueError("num_tokens must be non-negative")
+
     model = _normalize_model_for_pricing(model)
     if model not in TOKEN_COSTS:
         raise KeyError(