diff --git a/src/physrisk/risk_models/generic_risk_model.py b/src/physrisk/risk_models/generic_risk_model.py
index 5d76c89f..f60dfe9d 100644
--- a/src/physrisk/risk_models/generic_risk_model.py
+++ b/src/physrisk/risk_models/generic_risk_model.py
@@ -34,6 +34,7 @@
 )
 from physrisk.kernel.risk import Measure, MeasureKey, RiskMeasureCalculator
 from pint import UnitRegistry
+from physrisk.utils.units import needs_conversion
 
 ureg = UnitRegistry()
 
@@ -445,7 +446,7 @@ def calc_measure(
                 param = float(
                     np.interp(return_period, resp.return_periods, resp.intensities)
                 )
-                if resp.units != "default":
+                if needs_conversion(resp.units, bounds.units):
                     param = ureg.convert(param, resp.units, bounds.units)
             if math.isnan(param):
                 return Measure(
diff --git a/src/physrisk/utils/units.py b/src/physrisk/utils/units.py
new file mode 100644
index 00000000..32046d1f
--- /dev/null
+++ b/src/physrisk/utils/units.py
@@ -0,0 +1,33 @@
+UNITLESS = {"index", ""}
+
+
+def needs_conversion(source_units: str, target_units: str | None) -> bool:
+    """Return whether values should be converted between two unit labels.
+
+    Units marked as ``default`` and missing target units do not require
+    conversion. Equal units also do not require conversion. Unitless indicators (e.g. index)
+    cannot be converted to or from dimensional units, and attempting to do so raises an error.
+
+    Args:
+        source_units: Units attached to the source values.
+        target_units: Units expected by the target consumer, or ``None`` if
+            unspecified.
+
+    Returns:
+        ``True`` when unit conversion should be attempted, otherwise ``False``.
+
+    Raises:
+        ValueError: If source and target are provided, source is not ``default``, and only one is unitless.
+    """
+    if source_units == "default" or target_units in (None, "default"):
+        return False
+
+    if source_units == target_units:
+        return False
+
+    if source_units in UNITLESS or target_units in UNITLESS:
+        raise ValueError(
+            f"Conversion is not defined between incompatible units: {source_units} and {target_units}"
+        )
+
+    return True
diff --git a/src/physrisk/vulnerability_models/config_based_vuln_model_acute.py b/src/physrisk/vulnerability_models/config_based_vuln_model_acute.py
index b1f4dfc6..2faab572 100644
--- a/src/physrisk/vulnerability_models/config_based_vuln_model_acute.py
+++ b/src/physrisk/vulnerability_models/config_based_vuln_model_acute.py
@@ -39,6 +39,7 @@
 from physrisk.vulnerability_models.impact_function_selector import (
     ImpactFunctionSelector,
 )
+from physrisk.utils.units import needs_conversion
 
 ureg = UnitRegistry()
 logger = logging.getLogger(__name__)
@@ -152,7 +153,7 @@ def get_distributions(
                 "Future hazard curve is not monotonic; adjusting to ensure non-decreasing curve."
             )
 
-        conversion = curve.indicator_units is not None and future.units != "default"
+        conversion = needs_conversion(future.units, curve.indicator_units)
         if conversion:
             fut_intensities = ureg.convert(
                 fut_intensities, future.units, curve.indicator_units
diff --git a/tests/vulnerability_models/test_config_based_vulnerability.py b/tests/vulnerability_models/test_config_based_vulnerability.py
index 34e68a5e..cf57caaa 100644
--- a/tests/vulnerability_models/test_config_based_vulnerability.py
+++ b/tests/vulnerability_models/test_config_based_vulnerability.py
@@ -34,6 +34,7 @@
     RiverineInundation,
     Wind,
 )
+from physrisk.kernel.hazard_model import HazardEventDataResponse
 from physrisk.kernel.impact_distrib import ImpactType
 from physrisk.vulnerability_models.config_based_impact_curves import (
     PiecewiseLinearImpactCurve,
@@ -812,6 +813,42 @@ def test_create_vulnerability_model():
     )
 
 
+def test_config_based_vulnerability_accepts_index_units():
+    config_items = [
+        VulnerabilityConfigItem(
+            hazard_class="Fire",
+            asset_class="Asset",
+            asset_identifier="type=Generic,location=Generic",
+            indicator_id="susceptibility",
+            indicator_units="index",
+            impact_id="damage",
+            impact_units=None,
+            curve_type="indicator/piecewise_linear",
+            points_x=[0.0, 0.5, 1.0],
+            points_y=[0.0, 0.2, 1.0],
+        )
+    ]
+    factory = VulnerabilityModelsFactory(config=config_items)
+    vulnerability_models = factory.vulnerability_models()
+    model = vulnerability_models.vuln_model_for_asset_of_type(Asset)[0]
+    asset = Asset(location="Generic", latitude=0.0, longitude=0.0)
+    hazard_response = HazardEventDataResponse(
+        return_periods=np.array([10.0, 100.0, 1000.0]),
+        intensities=np.array([0.2, 0.6, 0.9]),
+        units="index",
+    )
+
+    vulnerability, event = model.get_distributions(asset, [hazard_response])
+
+    np.testing.assert_array_equal(
+        vulnerability.intensity_bins, np.array([0.2, 0.6, 0.9, 0.9])
+    )
+    np.testing.assert_array_almost_equal(
+        vulnerability.impact_bins, np.array([0.08, 0.36, 0.84, 0.84])
+    )
+    assert event.units == "index"
+
+
 @pytest.mark.skip("example, not test")
 def test_read_write_utilities(tmp_path):
     config = basic_vulnerability_config()