diff --git a/src/averagedistanceassembler.hh b/src/averagedistanceassembler.hh
index 56271d4dcc20169af73d28e9f6346c6042fda8d0..ff09c61e8aa835ded08115d7ed8eedda8de266d0 100644
--- a/src/averagedistanceassembler.hh
+++ b/src/averagedistanceassembler.hh
@@ -50,7 +50,7 @@ public:
                          Dune::FieldMatrix<double,size,size>& matrix) const
     {
         matrix = 0;
-        for (int i=0; i<coefficients_.size(); i++)
+        for (size_t i=0; i<coefficients_.size(); i++)
             matrix.axpy(weights_[i], TargetSpace::secondDerivativeOfDistanceSquaredWRTSecondArgument(coefficients_[i], x));
     }