diff --git a/dune/tectonic/localnonlinearity.hh b/dune/tectonic/localnonlinearity.hh
index 81c81552dd9af306a5ba44b8f10edb4d8130d9c6..d95e5724cce90c22a312ca03be97ce40a783816a 100644
--- a/dune/tectonic/localnonlinearity.hh
+++ b/dune/tectonic/localnonlinearity.hh
@@ -99,16 +99,20 @@ template <int dimension> class LocalNonlinearity {
     y += tmp;
   }
 
-  // TODO: do not evaluate at zero
   void upperGradient(VectorType const &x, VectorType &ret) const {
+    double const xnorm = x.two_norm();
+    assert(xnorm != 0);
+
     ret = x;
-    ret *= func_->rightDifferential(x.two_norm()) / x.two_norm();
+    ret *= func_->rightDifferential(xnorm) / xnorm;
   }
 
-  // TODO: do not evaluate at zero
   void lowerGradient(VectorType const &x, VectorType &ret) const {
+    double const xnorm = x.two_norm();
+    assert(xnorm != 0);
+
     ret = x;
-    ret *= func_->leftDifferential(x.two_norm()) / x.two_norm();
+    ret *= func_->leftDifferential(xnorm) / xnorm;
   }
 
   void directionalDomain(VectorType const &, VectorType const &,