diff --git a/perceptionmetrics/models/torch_segmentation.py b/perceptionmetrics/models/torch_segmentation.py
index 3b1a0476..72931e6e 100644
--- a/perceptionmetrics/models/torch_segmentation.py
+++ b/perceptionmetrics/models/torch_segmentation.py
@@ -413,7 +413,8 @@ def eval(
         if lut_ontology is not None:
             lut_ontology = torch.tensor(lut_ontology, dtype=torch.int64).to(self.device)
 
-        n_classes = len(eval_ontology)
+        # Class indices may be non-contiguous (e.g. RELLIS-3D dataset ontology)
+        n_classes = max(c["idx"] for c in eval_ontology.values()) + 1
 
         # Retrieve ignored label indices
         ignored_label_indices = []
diff --git a/tests/models/test_torch_segmentation.py b/tests/models/test_torch_segmentation.py
new file mode 100644
index 00000000..3cd30916
--- /dev/null
+++ b/tests/models/test_torch_segmentation.py
@@ -0,0 +1,93 @@
+import json
+from unittest.mock import MagicMock
+import warnings
+
+import numpy as np
+import pandas as pd
+from PIL import Image
+import pytest
+
+# skip these tests when unavailable
+torch = pytest.importorskip("torch")
+pytest.importorskip("torchvision")
+
+if isinstance(torch, MagicMock):
+    pytest.skip("real torch/torchvision required", allow_module_level=True)
+
+from perceptionmetrics.datasets.segmentation import ImageSegmentationDataset
+from perceptionmetrics.models.torch_segmentation import TorchImageSegmentationModel
+
+# Ground truth expressed in model indices (void, grass, tree)
+GT_MODEL_IDX = np.array([[0, 0, 1, 1], [0, 0, 1, 1], [2, 2, 2, 2], [2, 2, 2, 2]])
+
+
+class PerfectSegmenter(torch.nn.Module):
+    """Model whose logits always match the ground truth in model indices."""
+
+    def __init__(self):
+        super().__init__()
+        logits = torch.nn.functional.one_hot(torch.tensor(GT_MODEL_IDX), 3)
+        self.register_buffer("logits", logits.permute(2, 0, 1)[None].float())
+
+    def forward(self, x):
+        """Return the stored logits for every image in the batch.
+
+        :param x: Input image batch
+        :type x: torch.Tensor
+        :return: Logits with shape (batch, classes, height, width)
+        :rtype: torch.Tensor
+        """
+        return self.logits.expand(x.shape[0], -1, -1, -1)
+
+
+def build_model_and_dataset(tmp_path):
+    """Build a one-sample dataset with non-contiguous indices and a perfect model.
+
+    :param tmp_path: Temporary directory provided by pytest
+    :type tmp_path: pathlib.Path
+    :return: Model and dataset ready for evaluation
+    :rtype: Tuple[TorchImageSegmentationModel, ImageSegmentationDataset]
+    """
+    dataset_ontology = {"void": {"idx": 0}, "grass": {"idx": 3}, "tree": {"idx": 4}}
+    model_ontology = {"void": {"idx": 0}, "grass": {"idx": 1}, "tree": {"idx": 2}}
+
+    gt = np.array([0, 3, 4], dtype=np.uint8)[GT_MODEL_IDX]
+    Image.new("RGB", (4, 4)).save(tmp_path / "image.png")
+    Image.fromarray(gt).save(tmp_path / "label.png")
+    (tmp_path / "ontology.json").write_text(json.dumps(model_ontology))
+    (tmp_path / "cfg.json").write_text(json.dumps({"num_workers": 0}))
+
+    dataset = ImageSegmentationDataset(
+        pd.DataFrame(
+            {"image": ["image.png"], "label": ["label.png"], "split": ["test"]},
+            index=["sample"],
+        ),
+        str(tmp_path),
+        dataset_ontology,
+    )
+    model = TorchImageSegmentationModel(
+        PerfectSegmenter(),
+        str(tmp_path / "cfg.json"),
+        str(tmp_path / "ontology.json"),
+    )
+    return model, dataset
+
+
+# To verify that both translation directions work with non-contiguous dataset indices
+@pytest.mark.parametrize("direction", ["dataset_to_model", "model_to_dataset"])
+def test_eval_non_contiguous_dataset_indices(tmp_path, direction):
+    """Evaluate a perfect model in both translation directions.
+
+    :param tmp_path: Temporary directory provided by pytest
+    :type tmp_path: pathlib.Path
+    :param direction: Ontology translation direction
+    :type direction: str
+    """
+    model, dataset = build_model_and_dataset(tmp_path)
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")  # differing indices warning is expected
+        results = model.eval(dataset, split="test", translation_direction=direction)
+
+    assert results.loc["iou", ["void", "grass", "tree"]].tolist() == [1.0, 1.0, 1.0]
+    assert results.loc["iou", "macro"] == 1.0