diff --git a/kempnerforge/data/sampler.py b/kempnerforge/data/sampler.py
index 6b7281b..d8e6fc3 100644
--- a/kempnerforge/data/sampler.py
+++ b/kempnerforge/data/sampler.py
@@ -15,6 +15,27 @@
 from torch.utils.data import Dataset, Sampler
 
 
+def _validate_weights(weights: list[float], context: str) -> None:
+    """Fail fast on empty, negative, or all-zero weight lists.
+
+    The two normalization branches in ``MixtureSampler`` disagree on all-zero
+    input: the ``temperature == 1.0`` branch divides by ``sum(weights)`` and
+    raises ``ZeroDivisionError``; the ``temperature != 1.0`` branch clamps via
+    ``max(w, 1e-12)`` and silently produces uniform sampling. Reject both
+    cases up-front with a clear error so misconfigured phase transitions
+    surface immediately instead of crashing mid-run or drifting silently.
+    """
+    if not weights:
+        raise ValueError(f"{context}: weights list is empty")
+    if any(w < 0 for w in weights):
+        raise ValueError(f"{context}: weights must be non-negative, got {weights}")
+    if sum(weights) <= 0:
+        raise ValueError(
+            f"{context}: weights must sum to > 0 (at least one dataset must have "
+            f"weight > 0), got {weights}"
+        )
+
+
 class DistributedSampler(Sampler[int]):
     """Deterministic distributed sampler with skip-ahead support.
 
@@ -159,6 +180,8 @@ def __init__(
         self._dataset_sizes = [cumulative_sizes[i + 1] - cumulative_sizes[i] for i in range(n)]
         self._offsets = list(cumulative_sizes[:n])
 
+        _validate_weights(weights, "MixtureSampler(weights=...)")
+
         # Apply temperature scaling and normalize
         if temperature != 1.0:
             import math as _math
@@ -285,6 +308,8 @@ def update_weights(self, weights: list[float], temperature: float = 1.0) -> None
         if len(weights) != n:
             raise ValueError(f"Expected {n} weights, got {len(weights)}")
 
+        _validate_weights(weights, "MixtureSampler.update_weights")
+
         # Apply temperature scaling and normalize (same logic as __init__)
         if temperature != 1.0:
             import math as _math
diff --git a/tests/unit/test_mixing.py b/tests/unit/test_mixing.py
index 98fb677..b9e12f8 100644
--- a/tests/unit/test_mixing.py
+++ b/tests/unit/test_mixing.py
@@ -440,3 +440,100 @@ def test_dataset_idx_matches_sampler_intent(self, two_mmap_dirs):
         # All indices in ds_b range → dataset_idx=1
         for i in range(len(ds_a), len(mix)):
             assert mix[i]["dataset_idx"] == 1
+
+
+# ---------------------------------------------------------------------------
+# Zero-weight validation
+# ---------------------------------------------------------------------------
+
+
+class TestMixtureSamplerZeroWeights:
+    """Both normalization branches must reject all-zero weights up-front.
+
+    Previously, ``temperature == 1.0`` crashed with ZeroDivisionError at the
+    ``/`` inside __init__/update_weights, and ``temperature != 1.0`` silently
+    degraded to uniform sampling via the ``max(w, 1e-12)`` clamp. Both are
+    config errors and should raise the same ValueError with a clear message.
+    """
+
+    def test_init_rejects_all_zero_weights(self):
+        with pytest.raises(ValueError, match="sum to > 0"):
+            MixtureSampler(
+                cumulative_sizes=[0, 10, 20],
+                weights=[0.0, 0.0],
+                num_replicas=1,
+                rank=0,
+            )
+
+    def test_init_rejects_empty_weights(self):
+        with pytest.raises(ValueError, match="empty"):
+            MixtureSampler(
+                cumulative_sizes=[0],
+                weights=[],
+                num_replicas=1,
+                rank=0,
+            )
+
+    def test_init_rejects_negative_weights(self):
+        with pytest.raises(ValueError, match="non-negative"):
+            MixtureSampler(
+                cumulative_sizes=[0, 10, 20],
+                weights=[1.0, -0.5],
+                num_replicas=1,
+                rank=0,
+            )
+
+    def test_init_rejects_all_zero_with_temperature(self):
+        """Regression guard: temperature branch previously silently produced uniform."""
+        with pytest.raises(ValueError, match="sum to > 0"):
+            MixtureSampler(
+                cumulative_sizes=[0, 10, 20],
+                weights=[0.0, 0.0],
+                num_replicas=1,
+                rank=0,
+                temperature=2.0,
+            )
+
+    def test_update_weights_rejects_all_zero(self):
+        sampler = MixtureSampler(
+            cumulative_sizes=[0, 10, 20],
+            weights=[0.5, 0.5],
+            num_replicas=1,
+            rank=0,
+        )
+        with pytest.raises(ValueError, match="sum to > 0"):
+            sampler.update_weights([0.0, 0.0])
+
+    def test_update_weights_rejects_all_zero_with_temperature(self):
+        """Regression guard: the temperature != 1.0 branch must also reject
+        all-zero weights."""
+        sampler = MixtureSampler(
+            cumulative_sizes=[0, 10, 20],
+            weights=[0.5, 0.5],
+            num_replicas=1,
+            rank=0,
+            temperature=2.0,
+        )
+        with pytest.raises(ValueError, match="sum to > 0"):
+            sampler.update_weights([0.0, 0.0], temperature=2.0)
+
+    def test_update_weights_accepts_single_zero_with_nonzero_companion(self):
+        """A single dataset can be zeroed out as long as at least one stays positive."""
+        sampler = MixtureSampler(
+            cumulative_sizes=[0, 10, 20],
+            weights=[0.5, 0.5],
+            num_replicas=1,
+            rank=0,
+        )
+        sampler.update_weights([0.0, 1.0])  # must not raise
+        assert sampler._probs == [0.0, 1.0]
+
+    def test_update_weights_rejects_negative(self):
+        sampler = MixtureSampler(
+            cumulative_sizes=[0, 10, 20],
+            weights=[0.5, 0.5],
+            num_replicas=1,
+            rank=0,
+        )
+        with pytest.raises(ValueError, match="non-negative"):
+            sampler.update_weights([1.0, -0.2])