diff --git a/changelog_entry.yaml b/changelog_entry.yaml
index e69de29b..c3008017 100644
--- a/changelog_entry.yaml
+++ b/changelog_entry.yaml
@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    changed:
+      - Invalid enum values now raise ValueError instead of logging a warning and returning index 0. This prevents silent data corruption when incorrect enum strings are passed to simulations.
diff --git a/policyengine_core/enums/enum.py b/policyengine_core/enums/enum.py
index 3072d9a2..5b6e4b9e 100644
--- a/policyengine_core/enums/enum.py
+++ b/policyengine_core/enums/enum.py
@@ -85,14 +85,14 @@ def encode(cls, array: Union[EnumArray, np.ndarray]) -> EnumArray:
             # For non-matches, return 0 (first enum value) to match old np.select behaviour
             matches = sorted_names[positions] == array
             indices = np.where(matches, sorted_indices[positions], 0)
-            # Log warning for invalid values
+            # Raise error for invalid values to prevent silent data corruption
             invalid_mask = ~matches
             if np.any(invalid_mask):
                 invalid_values = np.unique(array[invalid_mask])
-                log.warning(
-                    f"Invalid values for enum {cls.__name__}: "
-                    f"{invalid_values.tolist()}. "
-                    f"These will be encoded as index 0."
+                valid_names = [item.name for item in cls]
+                raise ValueError(
+                    f"Invalid value(s) {invalid_values.tolist()} for enum "
+                    f"{cls.__name__}. Valid values are: {valid_names}"
                 )
         elif array.dtype.kind in {"i", "u"}:
             # Integer array - already indices
diff --git a/tests/core/enums/test_enum.py b/tests/core/enums/test_enum.py
index 6c8833b3..3605ef6d 100644
--- a/tests/core/enums/test_enum.py
+++ b/tests/core/enums/test_enum.py
@@ -1,6 +1,5 @@
 import pytest
 import numpy as np
-import logging
 from policyengine_core.enums.enum import Enum
 from policyengine_core.enums.enum_array import EnumArray
 
@@ -40,8 +39,8 @@ class Sample(Enum):
     assert encoded_array.dtype.kind == "i"
 
 
-def test_enum_encode_invalid_values_logs_warning(caplog):
-    """Test that encoding invalid enum string values logs a warning."""
+def test_enum_encode_invalid_values_raises_error():
+    """Test that encoding invalid enum string values raises ValueError."""
 
     class Sample(Enum):
         MAXWELL = "maxwell"
@@ -50,22 +49,18 @@ class Sample(Enum):
     # Array with invalid values mixed in
     array_with_invalid = np.array(["MAXWELL", "INVALID_VALUE", "DWORKIN"])
 
-    with caplog.at_level(logging.WARNING):
-        encoded = Sample.encode(array_with_invalid)
+    with pytest.raises(ValueError) as exc_info:
+        Sample.encode(array_with_invalid)
 
-    # Should still return an array (with 0 for invalid)
-    assert len(encoded) == 3
-    assert encoded[0] == Sample.MAXWELL.index
-    assert encoded[1] == 0  # Invalid defaults to 0
-    assert encoded[2] == Sample.DWORKIN.index
+    error_message = str(exc_info.value)
+    assert "INVALID_VALUE" in error_message
+    assert "Sample" in error_message
+    assert "MAXWELL" in error_message  # Valid values listed
+    assert "DWORKIN" in error_message  # Valid values listed
 
-    # Should have logged a warning
-    assert any("INVALID_VALUE" in record.message for record in caplog.records)
-    assert any("Sample" in record.message for record in caplog.records)
 
-
-def test_enum_encode_all_invalid_logs_warning(caplog):
-    """Test that encoding all invalid values logs a warning."""
+def test_enum_encode_all_invalid_raises_error():
+    """Test that encoding all invalid values raises ValueError."""
 
     class Sample(Enum):
         MAXWELL = "maxwell"
@@ -73,11 +68,29 @@ class Sample(Enum):
 
     all_invalid = np.array(["FOO", "BAR", "BAZ"])
 
-    with caplog.at_level(logging.WARNING):
-        encoded = Sample.encode(all_invalid)
+    with pytest.raises(ValueError) as exc_info:
+        Sample.encode(all_invalid)
+
+    error_message = str(exc_info.value)
+    # Should mention all unique invalid values
+    assert (
+        "FOO" in error_message
+        or "BAR" in error_message
+        or "BAZ" in error_message
+    )
+
+
+def test_enum_encode_empty_string_raises_error():
+    """Test that encoding empty strings raises ValueError."""
+
+    class Sample(Enum):
+        MAXWELL = "maxwell"
+        DWORKIN = "dworkin"
+
+    array_with_empty = np.array(["MAXWELL", "", "DWORKIN"])
 
-    # All should be 0
-    assert all(encoded == 0)
+    with pytest.raises(ValueError) as exc_info:
+        Sample.encode(array_with_empty)
 
-    # Should have logged warnings
-    assert len(caplog.records) > 0
+    # Empty string should be in the error message (represented as '')
+    assert "''" in str(exc_info.value) or '""' in str(exc_info.value)