diff --git a/dune/tectonic/myblockproblem.hh b/dune/tectonic/myblockproblem.hh
index 96fa97acd2a5cf29562b7d4e7aad2af9bb60ba07..352fdb0c5c1a2ecf9ffe8934f2a19f4c119b3b77 100644
--- a/dune/tectonic/myblockproblem.hh
+++ b/dune/tectonic/myblockproblem.hh
@@ -131,7 +131,8 @@ template <class MyConvexProblemTypeTEMPLATE> class MyBlockProblem {
     Interval<double> D;
     psi.subDiff(0, D);
     if (D[1] > 0) {
-      assert(false); // NOTE: Numerical instability can actually get us here
+      // NOTE: Numerical instability can actually get us here
+      assert(abs(D[1]) < 1e-15);
       return 0;
     }