diff --git a/dumux/assembly/fvassembler.hh b/dumux/assembly/fvassembler.hh
index e90d33ee9104451bb1878a2a2f37a4736fa66eff..8522297f355792220fc74b8eb01bd3f884b8b2c6 100644
--- a/dumux/assembly/fvassembler.hh
+++ b/dumux/assembly/fvassembler.hh
@@ -489,6 +489,8 @@ private:
         {
             if (enableMultithreading_)
             {
+                assert(elementSets_.size() > 0);
+
                 // make this element loop run in parallel
                 // for this we have to color the elements so that we don't get
                 // race conditions when writing into the global matrix