diff --git a/dune/gfe/riemanniantrsolver.cc b/dune/gfe/riemanniantrsolver.cc
index 12203ab29b32d47ba98bc536d967717a71f747d2..236b732c7cca7b891b1254ca168e35a9e41b2bb1 100644
--- a/dune/gfe/riemanniantrsolver.cc
+++ b/dune/gfe/riemanniantrsolver.cc
@@ -396,7 +396,7 @@ void RiemannianTrustRegionSolver<GridType,TargetSpace>::solve()
         // ////////////////////////////////////////////////////
 
         SolutionType newIterate = x_;
-        for (int j=0; j<newIterate.size(); j++)
+        for (size_t j=0; j<newIterate.size(); j++)
             newIterate[j] = TargetSpace::exp(newIterate[j], corr[j]);
 
         double energy    = assembler_->computeEnergy(newIterate);