diff --git a/dune/gfe/gramschmidtsolver.hh b/dune/gfe/gramschmidtsolver.hh
index 3623ff6b067607e5eb0d657f89ab272d5eb7ff78..663fd1814bd9188d3453386439852e537feb6938 100644
--- a/dune/gfe/gramschmidtsolver.hh
+++ b/dune/gfe/gramschmidtsolver.hh
@@ -42,7 +42,7 @@ class GramSchmidtSolver
 
     field_type energyScalarProduct = matrix.energyScalarProduct(vi,vj);
 
-    for (int i=0; i<vj.size(); i++)
+    for (size_t i=0; i<vj.size(); i++)
       vj[i] -= energyScalarProduct * vi[i];
 
   }