diff --git a/openevolve/llm/ensemble.py b/openevolve/llm/ensemble.py
index 1d912971f..c2b68cf14 100644
--- a/openevolve/llm/ensemble.py
+++ b/openevolve/llm/ensemble.py
@@ -4,6 +4,7 @@
 
 import asyncio
 import logging
+import math
 import random
 from typing import Dict, List, Optional, Tuple
 
@@ -54,13 +55,22 @@ class LLMEnsemble:
     def __init__(self, models_cfg: List[LLMModelConfig]):
         self.models_cfg = models_cfg
 
-        # Initialize models from the configuration
-        self.models = [_create_model(model_cfg) for model_cfg in models_cfg]
-
-        # Extract and normalize model weights
+        # Validate weights before initializing clients, which may have side effects.
         self.weights = [model.weight for model in models_cfg]
+        if any(not math.isfinite(weight) or weight < 0 for weight in self.weights):
+            raise ValueError("LLM ensemble weights must be finite and non-negative")
         total = sum(self.weights)
-        self.weights = [w / total for w in self.weights]
+        if self.weights and total <= 0:
+            raise ValueError("LLM ensemble must contain at least one model with a positive weight")
+        if not math.isfinite(total):
+            # Finite weights can still overflow when added. Rescale only this
+            # case so ordinary normalization and seeded sampling stay unchanged.
+            scale = max(self.weights)
+            self.weights = [weight / scale for weight in self.weights]
+            total = sum(self.weights)
+        self.weights = [weight / total for weight in self.weights]
+
+        self.models = [_create_model(model_cfg) for model_cfg in models_cfg]
 
         # Set up random state for deterministic model selection
         self.random_state = random.Random()
diff --git a/tests/test_ensemble_weight_validation.py b/tests/test_ensemble_weight_validation.py
new file mode 100644
index 000000000..7a23874d8
--- /dev/null
+++ b/tests/test_ensemble_weight_validation.py
@@ -0,0 +1,84 @@
+"""Ensemble weights must remain usable before any model client is initialized."""
+
+import math
+import unittest
+
+from openevolve.config import LLMModelConfig
+from openevolve.llm.ensemble import LLMEnsemble
+
+
+class OfflineClient:
+    def __init__(self, config):
+        self.model = config.name
+
+
+class TestEnsembleWeightValidation(unittest.TestCase):
+    def make_configs(self, weights, init_client=OfflineClient):
+        return [
+            LLMModelConfig(name=f"model-{index}", weight=weight, init_client=init_client)
+            for index, weight in enumerate(weights)
+        ]
+
+    def test_rejects_invalid_weights_before_initializing_clients(self):
+        invalid = (
+            [float("nan"), 1.0],
+            [float("inf"), 1.0],
+            [float("-inf"), 1.0],
+            [-1.0, 2.0],
+            [-1.0, -2.0],
+        )
+        for weights in invalid:
+            with self.subTest(weights=weights):
+                initialized = []
+
+                def initialize(config):
+                    initialized.append(config.name)
+                    return OfflineClient(config)
+
+                with self.assertRaisesRegex(ValueError, "finite and non-negative"):
+                    LLMEnsemble(self.make_configs(weights, initialize))
+                self.assertEqual(initialized, [])
+
+    def test_rejects_an_ensemble_with_no_positive_weight(self):
+        for weights in ([0.0], [0.0, 0.0]):
+            with self.subTest(weights=weights):
+                initialized = []
+
+                def initialize(config):
+                    initialized.append(config.name)
+                    return OfflineClient(config)
+
+                with self.assertRaisesRegex(ValueError, "positive weight"):
+                    LLMEnsemble(self.make_configs(weights, initialize))
+                self.assertEqual(initialized, [])
+
+    def test_preserves_empty_placeholder_ensembles(self):
+        ensemble = LLMEnsemble([])
+        self.assertEqual(ensemble.models, [])
+        self.assertEqual(ensemble.weights, [])
+
+    def test_normalizes_finite_weights_when_their_sum_overflows(self):
+        configs = self.make_configs([1e308, 1e308, 0.0])
+        ensemble = LLMEnsemble(configs)
+        self.assertEqual(ensemble.weights, [0.5, 0.5, 0.0])
+        ensemble.random_state.seed(42)
+        sampled = {ensemble._sample_model().model for _ in range(30)}
+        self.assertEqual(sampled, {"model-0", "model-1"})
+        self.assertEqual([config.weight for config in configs], [1e308, 1e308, 0.0])
+
+    def test_preserves_ratios_for_unequal_overflowing_weights(self):
+        ensemble = LLMEnsemble(self.make_configs([1e308, 5e307, 5e307]))
+        self.assertEqual(ensemble.weights, [0.5, 0.25, 0.25])
+        self.assertTrue(all(math.isfinite(weight) for weight in ensemble.weights))
+        self.assertEqual(sum(ensemble.weights), 1.0)
+
+    def test_preserves_ordinary_normalization_and_disabled_models(self):
+        for weights in ([1.0, 3.0, 0.0], [0.0, 1.0], [0.3, 0.3, 0.3], [1, 2, 3]):
+            with self.subTest(weights=weights):
+                ensemble = LLMEnsemble(self.make_configs(weights))
+                self.assertEqual(ensemble.weights, [weight / sum(weights) for weight in weights])
+                self.assertEqual(len(ensemble.models), len(weights))
+
+    def test_preserves_subnormal_positive_weights(self):
+        ensemble = LLMEnsemble(self.make_configs([5e-324, 5e-324]))
+        self.assertEqual(ensemble.weights, [0.5, 0.5])