diff --git a/src/mynonlinearity.cc b/src/mynonlinearity.cc
index faee61e3f06124635e073709d8cbdff3f1fe853f..4b95a4f5d7b5673afa909da6dbdc857b7b8825af 100644
--- a/src/mynonlinearity.cc
+++ b/src/mynonlinearity.cc
@@ -23,6 +23,7 @@ class MyNonlinearity {
   typedef SmallMatrix MatrixType;
 
   // directional subdifferential: at u on the line u + t*v
+  // 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)) {