Return None rather than KeyError, for property_data without a thermo doc in get_entries_in_chemsys

mp_api/client/mprester.py

@@ -1376,8 +1376,19 @@ def get_entries_in_chemsys(
                         str(doc["material_id"]): {p: doc[p] for p in property_data}
                         for doc in docs
                     }
+                    no_doc = dict.fromkeys(property_data)
+                    missing = []
                     for entry in entries:  # served entries carry `material_id`
-                        entry.data |= props[str(entry.data["material_id"])]
+                        material_id = str(entry.data["material_id"])
+                        if material_id not in props:
+                            missing.append(material_id)
+                        entry.data |= props.get(material_id, no_doc)
+                    if missing:  # e.g. materials removed since an older `db_version`
+                        logger.warning(
+                            f"No thermo document was found for {', '.join(missing)} in "
+                            f"database version {self.current_db_version} (used for REST "
+                            "queries), so their `property_data` fields are None."
+                        )
 
             if entries is None:
                 # MP has no pre-built diagram for this system, so redo the mixing here as MP does when

tests/client/test_mprester.py

@@ -436,6 +436,27 @@ def test_get_entries_in_chemsys_decorated_served_pd(self, mpr):
             assert entry.energy_per_atom == pytest.approx(energy, abs=1e-8)
             assert entry.composition.reduced_formula == formula
 
+    def test_get_entries_in_chemsys_decorated_served_pd_missing_docs(self, caplog):
+        """
+        With an older ``db_version``, the phase diagram is read from that version but
+        ``property_data`` from the REST API (current version), which lacks some of its
+        materials, e.g. ``mp-1275385`` in the v2026.04.13 Co-O phase diagram. These get
+        None rather than raising a ``KeyError``.
+        """
+        with MPRester(db_version="2026.04.13") as mpr:
+            with caplog.at_level(logging.WARNING, logger="mp_api.client"):
+                entries = mpr.get_entries_in_chemsys(
+                    "Co-O", property_data=["energy_above_hull"]
+                )
+        missing = {
+            str(e.data["material_id"])
+            for e in entries
+            if e.data["energy_above_hull"] is None
+        }
+        assert "mp-1275385" in missing
+        assert len(missing) < len(entries)
+        assert "No thermo document was found for mp-1275385" in caplog.text
+
     @pytest.mark.skipif(
         contribs_client is None,
         reason="`pip install 'mp-api[contribs]'` to use pourbaix functionality.",