diff --git a/backend/consensus/vrf.py b/backend/consensus/vrf.py
index 55c953600..6135e0d2d 100644
--- a/backend/consensus/vrf.py
+++ b/backend/consensus/vrf.py
@@ -6,18 +6,26 @@
 def get_validators_for_transaction(
     nodes: list[dict],
     num_validators: int | None = None,
-    rng=np.random.default_rng(seed=int(datetime.now().timestamp())),
+    rng: np.random.Generator | None = None,
 ) -> list[dict]:
     """
     Returns subset of validators for a transaction.
     The selelction and order is given by a random sampling based on the stake of the validators.
     """
+    if rng is None:
+        rng = np.random.default_rng(seed=int(datetime.now().timestamp()))
+
     if num_validators is None:
         num_validators = DEFAULT_VALIDATORS_COUNT
 
     num_validators = min(num_validators, len(nodes))
 
     total_stake = sum(validator["stake"] for validator in nodes)
+    if total_stake == 0:
+        raise ValueError(
+            "Cannot select validators: all stakes are zero. "
+            "At least one validator must have a positive stake."
+        )
     probabilities = [validator["stake"] / total_stake for validator in nodes]
 
     selected_validators = rng.choice(
diff --git a/tests/unit/test_vrf.py b/tests/unit/test_vrf.py
index 95473770e..4760b3d35 100644
--- a/tests/unit/test_vrf.py
+++ b/tests/unit/test_vrf.py
@@ -1,5 +1,6 @@
 from backend.consensus.vrf import get_validators_for_transaction
 from unittest.mock import Mock
+import pytest
 
 
 def list_of_dicts_to_set(list_of_dicts: list[dict]) -> set:
@@ -66,3 +67,21 @@ def choice_mock(a, p, size, replace):
 
     rng.choice.assert_called_once()
     assert validators == [{"stake": 3}, {"stake": 2}, {"stake": 1}]
+
+
+def test_get_validators_zero_stake_raises():
+    """When all stakes are zero, selection should raise ValueError."""
+    nodes = [{"stake": 0}, {"stake": 0}]
+    with pytest.raises(ValueError, match="all stakes are zero"):
+        get_validators_for_transaction(nodes, 1)
+
+
+def test_get_validators_rng_not_shared_between_calls():
+    """Each call without an explicit rng should get a fresh generator."""
+    nodes = [{"stake": 1}, {"stake": 2}, {"stake": 3}]
+    # Call twice without passing rng — the default must be created fresh each time.
+    v1 = get_validators_for_transaction(nodes, 1)
+    v2 = get_validators_for_transaction(nodes, 1)
+    # Both should succeed (no shared mutable state corruption).
+    assert len(v1) == 1
+    assert len(v2) == 1