Raise a clear error when a model result has no data at the observation

src/modelskill/matching.py

@@ -382,8 +382,6 @@ def _match_single_obs(
 
 
 def _get_global_start_end(idxs: Iterable[pd.DatetimeIndex]) -> Period:
-    assert all([len(x) > 0 for x in idxs]), "All datetime indices must be non-empty"
-
     starts = [x[0] for x in idxs]
     ends = [x[-1] for x in idxs]
 
@@ -408,6 +406,22 @@ def _check_timezone_compatibility(
             )
 
 
+def _check_model_results_not_empty(
+    observation: Observation,
+    raw_mod_data: Mapping[
+        str,
+        PointModelResult | TrackModelResult | VerticalModelResult | NodeModelResult,
+    ],
+) -> None:
+    for name, mr in raw_mod_data.items():
+        if len(mr.time) == 0:
+            raise ValueError(
+                f"Model result '{name}' has no data at observation '{observation.name}'. "
+                "Check that the observation is inside the model domain and that the "
+                "model has valid values there (e.g. the location is not dry)."
+            )
+
+
 def _at_observation(mr: NodeModelResult, obs: TimeSeries) -> NodeModelResult:
     """The extracted model result, placed where the observation sits.
 
@@ -431,6 +445,7 @@ def _match_space_time(
     obs_no_overlap: Literal["ignore", "error", "warn"],
 ) -> xr.Dataset | None:
     _check_timezone_compatibility(observation, raw_mod_data)
+    _check_model_results_not_empty(observation, raw_mod_data)
 
     idxs = [m.time for m in raw_mod_data.values()]
     period = _get_global_start_end(idxs)

tests/test_match.py

@@ -963,6 +963,23 @@ def test_match_raises_clear_error_on_tz_aware_vs_naive_mismatch():
         ms.match(obs, mr)
 
 
+def test_match_raises_clear_error_when_model_result_is_empty():
+    """An empty extraction (e.g. point outside domain or dry) must raise a clear error, not a bare assert."""
+    obs = ms.PointObservation(
+        data=pd.Series(
+            [1.0, 2.0, 3.0], index=pd.date_range("2020-01-01", periods=3, freq="h")
+        ),
+        name="Station",
+    )
+    mr = ms.PointModelResult(
+        data=pd.Series([], dtype=float, index=pd.DatetimeIndex([])),
+        name="Model",
+    )
+
+    with pytest.raises(ValueError, match="'Model' has no data.*'Station'"):
+        ms.match(obs, mr)
+
+
 def test_multiple_models_same_name(tmp_path: Path) -> None:
     obs = ms.PointObservation(
         "tests/testdata/SW/HKNA_Hm0.dfs0", item=0, x=4.2420, y=52.6887, name="HKNA"