diff --git a/SU2_CFD/src/fluid/CMutationTCLib.cpp b/SU2_CFD/src/fluid/CMutationTCLib.cpp
index 32c80f8151ac..132363eeaa5a 100644
--- a/SU2_CFD/src/fluid/CMutationTCLib.cpp
+++ b/SU2_CFD/src/fluid/CMutationTCLib.cpp
@@ -59,8 +59,15 @@ CMutationTCLib::CMutationTCLib(const CConfig* config, unsigned short val_nDim):
 
   for(iSpecies = 0; iSpecies < nSpecies; iSpecies++) MolarMass[iSpecies] = 1000* mix->speciesMw(iSpecies); // x1000 to have Molar Mass in kg/kmol
 
-  if (mix->hasElectrons()) { nHeavy = nSpecies-1; nEl = 1; }
-  else                     { nHeavy = nSpecies;   nEl = 0; }
+  if (mix->hasElectrons()) { 
+    if (config->GetViscous()) {
+      SU2_MPI::Error("Ionization is not yet operational for a viscous flow in the NEMO solver.", CURRENT_FUNCTION);
+    } else {
+      nHeavy = nSpecies-1;
+      nEl = 1;
+    }
+  }
+  else { nHeavy = nSpecies;   nEl = 0; }
 
 }