diff --git a/libsrc/meshing/improve2.cpp b/libsrc/meshing/improve2.cpp
index b215bfd7..c46df8a0 100644
--- a/libsrc/meshing/improve2.cpp
+++ b/libsrc/meshing/improve2.cpp
@@ -339,6 +339,9 @@ namespace netgen
             if (swapped[t1])
               return;
 
+            if(mesh[t1].GetNP() != 3)
+              return;
+
             if (multithread.terminate)
               throw NgException ("Meshing stopped");