diff --git a/tests/test_negative_lambda_reg.py b/tests/test_negative_lambda_reg.py
new file mode 100644
index 0000000..411d2fa
--- /dev/null
+++ b/tests/test_negative_lambda_reg.py
@@ -0,0 +1,27 @@
+import numpy as np
+import pytest
+
+from transformer_instant.elm import elm_fit
+from transformer_instant.linalg import ridge_fit_closed_form
+from transformer_instant.utils import krr_fit
+
+
+def test_ridge_negative_lambda_reg():
+    X = np.random.randn(4, 3)
+    y = np.random.randn(4, 1)
+    with pytest.raises(ValueError):
+        ridge_fit_closed_form(X, y, lambda_reg=-0.1)
+
+
+def test_krr_negative_lambda_reg():
+    X = np.random.randn(4, 3)
+    y = np.random.randn(4, 1)
+    with pytest.raises(ValueError):
+        krr_fit(X, y, lambda_reg=-0.1)
+
+
+def test_elm_negative_lambda_reg():
+    X = np.random.randn(4, 3)
+    y = np.random.randn(4, 1)
+    with pytest.raises(ValueError):
+        elm_fit(X, y, lambda_reg=-0.1)
diff --git a/transformer_instant/elm.py b/transformer_instant/elm.py
index eeb9a56..930c646 100644
--- a/transformer_instant/elm.py
+++ b/transformer_instant/elm.py
@@ -29,6 +29,9 @@ def elm_fit(
 ) -> dict[str, Array | float | int | str]:
     X = np.asarray(X, dtype=np.float64)
     Y = np.asarray(Y, dtype=np.float64)
+    lambda_reg = float(lambda_reg)
+    if lambda_reg < 0.0:
+        raise ValueError("lambda_reg must be non-negative")
     n, d = X.shape
     rng = np.random.default_rng(random_seed)
     W_hidden = rng.normal(
@@ -42,7 +45,7 @@ def elm_fit(
     else:
         raise ValueError("Unsupported activation: choose 'tanh' or 'relu'")
     H = act(cast(Array, (X @ W_hidden + b_hidden).astype(np.float64)))
-    if H.shape[0] == H.shape[1] and float(lambda_reg) <= 1e-8:
+    if H.shape[0] == H.shape[1] and lambda_reg <= 1e-8:
         try:
             beta = cast(Array, np.linalg.solve(H, Y).astype(np.float64))
         except Exception:
@@ -52,7 +55,7 @@ def elm_fit(
     else:
         HtH = H.T @ H
         if lambda_reg > 0.0:
-            HtH = HtH + float(lambda_reg) * np.eye(HtH.shape[0], dtype=HtH.dtype)
+            HtH = HtH + lambda_reg * np.eye(HtH.shape[0], dtype=HtH.dtype)
         Hty = H.T @ Y
         beta = solve_spd(HtH, Hty)
     return {
@@ -61,7 +64,7 @@ def elm_fit(
         "beta": beta,
         "activation": activation,
         "weight_scale": float(weight_scale),
-        "lambda_reg": float(lambda_reg),
+        "lambda_reg": lambda_reg,
         "hidden_units": int(hidden_units),
     }
 
diff --git a/transformer_instant/linalg.py b/transformer_instant/linalg.py
index 8863429..f70441d 100644
--- a/transformer_instant/linalg.py
+++ b/transformer_instant/linalg.py
@@ -45,6 +45,9 @@ def ridge_fit_closed_form(X: Array, Y: Array, lambda_reg: float = 1e-3) -> Array
     """
     X = np.asarray(X, dtype=np.float64)
     Y = np.asarray(Y, dtype=np.float64)
+    lambda_reg = float(lambda_reg)
+    if lambda_reg < 0.0:
+        raise ValueError("lambda_reg must be non-negative")
     if Y.ndim == 1:
         Y = Y.reshape(-1, 1)
         squeeze = True
@@ -53,7 +56,7 @@ def ridge_fit_closed_form(X: Array, Y: Array, lambda_reg: float = 1e-3) -> Array
     n, d = X.shape
     xtx = X.T @ X
     if lambda_reg > 0.0:
-        xtx = xtx + float(lambda_reg) * np.eye(d, dtype=xtx.dtype)
+        xtx = xtx + lambda_reg * np.eye(d, dtype=xtx.dtype)
     xty = X.T @ Y
     W = solve_spd(xtx, xty)
     if squeeze and W.ndim == 2 and W.shape[1] == 1:
diff --git a/transformer_instant/utils.py b/transformer_instant/utils.py
index ae30824..0732696 100644
--- a/transformer_instant/utils.py
+++ b/transformer_instant/utils.py
@@ -26,13 +26,16 @@ def krr_fit(
     """
     X = np.asarray(X, dtype=np.float64)
     Y = np.asarray(Y, dtype=np.float64)
+    lambda_reg = float(lambda_reg)
+    if lambda_reg < 0.0:
+        raise ValueError("lambda_reg must be non-negative")
     if kernel != "rbf":
         raise ValueError("Only 'rbf' kernel supported in krr_fit")
     # Build symmetric Gram with self-consistent diagonal for better precision
     K = rbf_cross_kernel(X, X, length_scale=length_scale, variance=variance)
     K = 0.5 * (K + K.T)
     n = K.shape[0]
-    lambda_eff = 0.0 if float(lambda_reg) <= 1e-8 else float(lambda_reg)
+    lambda_eff = 0.0 if lambda_reg <= 1e-8 else lambda_reg
     A = K + lambda_eff * np.eye(n, dtype=K.dtype)
     alpha = solve_spd(A, Y)
     return {
@@ -41,7 +44,7 @@ def krr_fit(
         "kernel": kernel,
         "length_scale": float(length_scale),
         "variance": float(variance),
-        "lambda_reg": float(lambda_reg),
+        "lambda_reg": lambda_reg,
     }