diff --git a/interfaces/cython/cantera/test/test_reactor.py b/interfaces/cython/cantera/test/test_reactor.py
index fd07e653e..1c119a843 100644
--- a/interfaces/cython/cantera/test/test_reactor.py
+++ b/interfaces/cython/cantera/test/test_reactor.py
@@ -886,7 +886,7 @@ class TestReactorSensitivities(utilities.CanteraTest):
             self.assertNear(np.linalg.norm(S[Ns:K2,1]), 0.0, atol=1e-5)
             self.assertNear(np.linalg.norm(S[K2+Ns:,0]), 0.0, atol=1e-5)
 
-    def test_parameter_order1(self):
+    def _test_parameter_order1(self, reactorClass):
         # Single reactor, changing the order in which parameters are added
         gas = ct.Solution('h2o2.xml')
 
@@ -894,7 +894,7 @@ class TestReactorSensitivities(utilities.CanteraTest):
             net = ct.ReactorNet()
             gas.TPX = 900, 101325, 'H2:0.1, OH:1e-7, O2:0.1, AR:1e-5'
 
-            r = ct.IdealGasReactor(gas)
+            r = reactorClass(gas)
             net.add_reactor(r)
             return r, net
 
@@ -925,6 +925,12 @@ class TestReactorSensitivities(utilities.CanteraTest):
         for i,j in enumerate((2,1,3,0)):
             self.assertArrayNear(S1[:,i], S2[:,j])
 
+    def test_parameter_order1a(self):
+        self._test_parameter_order1(ct.IdealGasReactor)
+
+    def test_parameter_order1b(self):
+        self._test_parameter_order1(ct.IdealGasConstPressureReactor)
+
     def test_parameter_order2(self):
         # Multiple reactors, changing the order in which parameters are added
         gas = ct.Solution('h2o2.xml')
diff --git a/src/zeroD/ConstPressureReactor.cpp b/src/zeroD/ConstPressureReactor.cpp
index dc9a7f671..6d8f4d367 100644
--- a/src/zeroD/ConstPressureReactor.cpp
+++ b/src/zeroD/ConstPressureReactor.cpp
@@ -83,6 +83,7 @@ void ConstPressureReactor::initialize(doublereal t0)
         }
     }
     m_work.resize(maxnt);
+    std::sort(m_pnum.begin(), m_pnum.end());
     m_init = true;
 }
 
diff --git a/src/zeroD/IdealGasConstPressureReactor.cpp b/src/zeroD/IdealGasConstPressureReactor.cpp
index f618bab82..59e7bbb59 100644
--- a/src/zeroD/IdealGasConstPressureReactor.cpp
+++ b/src/zeroD/IdealGasConstPressureReactor.cpp
@@ -94,6 +94,7 @@ void IdealGasConstPressureReactor::initialize(doublereal t0)
         }
     }
     m_work.resize(maxnt);
+    std::sort(m_pnum.begin(), m_pnum.end());
     m_init = true;
 }