diff --git a/src/decman/core/store.py b/src/decman/core/store.py
index 0414a25..4ab0b77 100644
--- a/src/decman/core/store.py
+++ b/src/decman/core/store.py
@@ -17,7 +17,7 @@ class Store:
 
         if self._path.exists():
             with self._path.open("rt", encoding="utf-8") as file:
-                self._store = json.load(file)
+                self._store = json.load(file, object_hook=_decode_sets)
 
     def __getitem__(self, key: str) -> typing.Any:
         return self._store[key]
@@ -30,7 +30,7 @@ class Store:
 
     def ensure(self, key: str, default: typing.Any = None):
         if key not in self._store:
-            self._store = default
+            self._store[key] = default
 
     def __enter__(self) -> "Store":
         return self
@@ -54,7 +54,7 @@ class Store:
             dir=self._path.parent,
             delete=False,
         ) as tmp:
-            json.dump(self._store, tmp, indent=2)
+            json.dump(self._store, tmp, cls=_SetJSONEncoder, indent=2)
             tmp.flush()
             os.fsync(tmp.fileno())
 
@@ -62,3 +62,17 @@ class Store:
 
     def __repr__(self) -> str:
         return repr(self._store)
+
+
+class _SetJSONEncoder(json.JSONEncoder):
+    def default(self, obj: typing.Any) -> typing.Any:
+        if isinstance(obj, set):
+            # generic, works for any set value
+            return {"__type__": "set", "items": list(obj)}
+        return super().default(obj)
+
+
+def _decode_sets(obj: typing.Any) -> typing.Any:
+    if isinstance(obj, dict) and obj.get("__type__") == "set" and "items" in obj:
+        return set(obj["items"])
+    return obj
diff --git a/tests/test_decman_core_store.py b/tests/test_decman_core_store.py
index e895913..64679e5 100644
--- a/tests/test_decman_core_store.py
+++ b/tests/test_decman_core_store.py
@@ -101,3 +101,29 @@ def test_repr_matches_underlying_dict(tmp_path: Path) -> None:
 
     expected = repr({"foo": "bar", "number": 123})
     assert repr(store) == expected
+
+
+def test_store_persists_sets(tmp_path: Path) -> None:
+    path = tmp_path / "store.json"
+
+    # initial write with sets
+    store = Store(path)
+    store["units"] = {"a.service", "b.service"}
+    store["user_units"] = {"alice": {"u1.service", "u2.service"}}
+    store.save()
+
+    # raw JSON should be set-encoded, not fail json.dump
+    raw = json.loads(path.read_text(encoding="utf-8"))
+    assert raw["units"]["__type__"] == "set"
+    assert set(raw["units"]["items"]) == {"a.service", "b.service"}
+
+    assert raw["user_units"]["alice"]["__type__"] == "set"
+    assert set(raw["user_units"]["alice"]["items"]) == {"u1.service", "u2.service"}
+
+    # reloading via Store must restore actual set objects
+    reloaded = Store(path)
+    assert reloaded["units"] == {"a.service", "b.service"}
+    assert isinstance(reloaded["units"], set)
+
+    assert reloaded["user_units"]["alice"] == {"u1.service", "u2.service"}
+    assert isinstance(reloaded["user_units"]["alice"], set)