diff --git a/interfaces/cython/cantera/thermo.pyx b/interfaces/cython/cantera/thermo.pyx
index 2a3cee2e1..df15b5a59 100644
--- a/interfaces/cython/cantera/thermo.pyx
+++ b/interfaces/cython/cantera/thermo.pyx
@@ -920,11 +920,13 @@ cdef class PureFluid(ThermoPhase):
         def __get__(self):
             return self.T, self.X
         def __set__(self, values):
-            self.thermo.setState_Tsat(values[0], values[1])
+            T = values[0] if values[0] is not None else self.T
+            self.thermo.setState_Tsat(T, values[1])
 
     property PX:
         """Get/Set the pressure and vapor fraction of a two-phase state."""
         def __get__(self):
             return self.P, self.X
         def __set__(self, values):
-            self.thermo.setState_Psat(values[0], values[1])
+            P = values[0] if values[0] is not None else self.P
+            self.thermo.setState_Psat(P, values[1])