diff --git a/src/db/db/dbTriangles.cc b/src/db/db/dbTriangles.cc
index 42e7ae0f7..4261233dc 100644
--- a/src/db/db/dbTriangles.cc
+++ b/src/db/db/dbTriangles.cc
@@ -1315,7 +1315,7 @@ Triangles::join_edges (std::vector<db::TriangleEdge *> &edges)
 void
 Triangles::constrain (const std::vector<std::vector<db::Vertex *> > &contours)
 {
-  assert (! m_is_constrained);
+  tl_assert (! m_is_constrained);
 
   std::vector<std::pair<db::DEdge, std::vector<db::TriangleEdge *> > > resolved_edges;