diff --git a/interfaces/cython/cantera/composite.py b/interfaces/cython/cantera/composite.py
index 79f2a781c6..1d0caa0861 100644
--- a/interfaces/cython/cantera/composite.py
+++ b/interfaces/cython/cantera/composite.py
@@ -195,6 +195,8 @@ class Quantity:
         >>> q3.P
         101325.0
     """
+    __slots__ = ("state", "_phase", "_id", "mass", "constant")
+
     def __init__(self, phase, mass=None, moles=None, constant='UV'):
         self.state = phase.TDY
         self._phase = phase
diff --git a/interfaces/cython/cantera/test/test_thermo.py b/interfaces/cython/cantera/test/test_thermo.py
index 6d5cbcd9a7..65bc5cedf1 100644
--- a/interfaces/cython/cantera/test/test_thermo.py
+++ b/interfaces/cython/cantera/test/test_thermo.py
@@ -1445,6 +1445,11 @@ def test_equilibrate(self):
         q1.equilibrate('HP')
         self.assertNear(q1.T, T2)
 
+    def test_invalid_setter(self):
+        q1 = ct.Quantity(self.gas, mass =3)
+        with self.assertRaises(AttributeError):
+            q1.HPQ = self.gas.H, self.gas.P, 1
+
     def test_incompatible(self):
         gas2 = ct.Solution('h2o2.yaml', transport_model=None)
         q1 = ct.Quantity(self.gas)