diff --git a/include/cantera/zeroD/FlowReactor.h b/include/cantera/zeroD/FlowReactor.h
index 6b163cbd6..d13ae10cd 100644
--- a/include/cantera/zeroD/FlowReactor.h
+++ b/include/cantera/zeroD/FlowReactor.h
@@ -140,7 +140,7 @@ protected:
     //! Density [kg/m^3]. First component of the state vector.
     double m_rho = NAN;
     //! Axial velocity [m/s]. Second component of the state vector.
-    double m_u = NAN;
+    double m_u = -1.0;
     //! Pressure [Pa]. Third component of the state vector.
     double m_P = NAN;
     //! Temperature [K]. Fourth component of the state vector.
diff --git a/src/zeroD/FlowReactor.cpp b/src/zeroD/FlowReactor.cpp
index 2d034fed2..6c0ffb6db 100644
--- a/src/zeroD/FlowReactor.cpp
+++ b/src/zeroD/FlowReactor.cpp
@@ -29,6 +29,11 @@ void FlowReactor::getStateDae(double* y, double* ydot)
     // set the first component to the initial density
     y[0] = m_rho;
 
+    if (m_u < 0) {
+        throw CanteraError("FlowReactor::getStateDae",
+            "Set mass flow rate before initializing reactor");
+    }
+
     // set the second component to the initial speed
     y[1] = m_u;
 
diff --git a/test/python/test_reactor.py b/test/python/test_reactor.py
index a50428c1a..4bb8d37f7 100644
--- a/test/python/test_reactor.py
+++ b/test/python/test_reactor.py
@@ -1499,6 +1499,7 @@ class TestFlowReactor(utilities.CanteraTest):
         surf = ct.Interface('methane_pox_on_pt.yaml', 'Pt_surf')
         gas = surf.adjacent['gas']
         r = ct.FlowReactor(gas)
+        r.mass_flow_rate = 0.1
         rsurf = ct.ReactorSurface(surf, r)
         sim = ct.ReactorNet([r])
         sim.initialize()
@@ -1530,6 +1531,14 @@ class TestFlowReactor2(utilities.CanteraTest):
         sim = ct.ReactorNet([r])
         return r, rsurf, sim
 
+    def test_no_mass_flow_rate(self):
+        surf, gas = self.import_phases()
+        r = ct.FlowReactor(gas)
+        rsurf = ct.ReactorSurface(surf, r)
+        sim = ct.ReactorNet([r])
+        with pytest.raises(ct.CanteraError, match="mass flow rate"):
+            sim.initialize()
+
     def test_mixed_reactor_types(self):
         surf, gas = self.import_phases()
         r1 = ct.FlowReactor(gas)