diff --git a/src/mynonlinearity.hh b/src/mynonlinearity.hh
index 30185b79e90c37861e328d71c487b04dca5653f6..d4e4bc7496506251192630844a91481733b1370f 100644
--- a/src/mynonlinearity.hh
+++ b/src/mynonlinearity.hh
@@ -32,7 +32,7 @@ class MyNonlinearity {
   // u and v are assumed to be non-zero
   void directionalSubDiff(VectorType const u, VectorType const v,
                           Interval<double> &D) const {
-    if (u == SmallVector(0.0)) {
+    if (u.two_norm() == 0) {
       D[0] = D[1] = func_.rightDifferential(0) * v.two_norm();
       return;
     }