diff --git a/dune/gfe/vtkreader.hh b/dune/gfe/vtkreader.hh
index 37954a9744a45b9867f331eb52feb933cd053374..566deead39a87ad62ed886dadd5c153921f5ab04 100644
--- a/dune/gfe/vtkreader.hh
+++ b/dune/gfe/vtkreader.hh
@@ -38,7 +38,7 @@ public:
     Dune::GeometryType triangle;
     triangle.makeTriangle();
 
-    for (int i=0; i<vtkFile.cellConnectivity_.size(); i+=3)
+    for (size_t i=0; i<vtkFile.cellConnectivity_.size(); i+=3)
     {
       factory.insertElement(triangle, {vtkFile.cellConnectivity_[i],
                                        vtkFile.cellConnectivity_[i+1],