diff --git a/dune/solvers/iterationsteps/blockgsstep.cc b/dune/solvers/iterationsteps/blockgsstep.cc
index cfa49b64cf77b62ae6d48d61a2fe68713f524a09..dacb54608baeb2d1566b115a22922d898c8e3fd3 100644
--- a/dune/solvers/iterationsteps/blockgsstep.cc
+++ b/dune/solvers/iterationsteps/blockgsstep.cc
@@ -44,11 +44,11 @@ void BlockGSStep<MatrixType, DiscFuncType, BitVectorType>::iterate()
     assert(this->ignoreNodes_ != NULL);
 
     if (gs_type_ != Direction::BACKWARD)
-        for (int i=0; i<this->x_->size(); i++)
+        for (std::size_t i=0; i<this->x_->size(); i++)
             iterate_step(i);
 
     if (gs_type_ != Direction::FORWARD)
-        for (int i=this->x_->size()-1; i>=0 && i<this->x_->size(); i--)
+        for (std::size_t i=this->x_->size()-1; i>=0 && i<this->x_->size(); i--)
             iterate_step(i);
 }