diff --git a/interfaces/cython/cantera/onedim.pyx b/interfaces/cython/cantera/onedim.pyx
index 9fbcafb78..05b2d8b73 100644
--- a/interfaces/cython/cantera/onedim.pyx
+++ b/interfaces/cython/cantera/onedim.pyx
@@ -1063,9 +1063,11 @@ class CounterflowDiffusionFlame(FlameBase):
 
         if stoich is None:
             if oxidizer == 'O2':
-                nH = self.gas.nAtoms(fuel, 'H')
-                nC = self.gas.nAtoms(fuel, 'C')
-                stoich = 1.0 * nC + 0.25 * nH
+                stoich = 0.0
+                if 'H' in self.gas.elementNames:
+                    stoich += 0.25 * self.gas.nAtoms(fuel, 'H')
+                if 'C' in self.gas.elementNames:
+                    stoich += self.gas.nAtoms(fuel, 'C')
             else:
                 raise Exception('oxidizer/fuel stoichiometric ratio must be '
                                 'specified since the oxidizer is not O2')