diff --git a/dune/tectonic/localfriction.hh b/dune/tectonic/localfriction.hh
index 57289d2dc0c4b8e9cde6f88f3a017d6e854a55d8..881def530bba82c88ca54034bd00ebc47f3ee330 100644
--- a/dune/tectonic/localfriction.hh
+++ b/dune/tectonic/localfriction.hh
@@ -59,7 +59,8 @@ template <size_t dimension> class LocalFriction {
                           Interval<double> &D) const {
     double const xnorm = x.two_norm();
     if (xnorm <= 0.0)
-      D[0] = D[1] = 0.0;
+      D[0] = D[1] = 0.0; // WARNING: we assume that the outer
+                         // function approaches zero superlinearly
     else
       D[0] = D[1] = func->differential(xnorm) * dotFirstNormalised(x, v);
   }