diff --git a/aos/vision/math/segment.h b/aos/vision/math/segment.h
index 706beaf..d6927cb 100644
--- a/aos/vision/math/segment.h
+++ b/aos/vision/math/segment.h
@@ -44,7 +44,7 @@
   }
 
   // Intersect two lines in a plane.
-  Vector<2> Intersect(const Segment<2> &other) {
+  Vector<2> Intersect(const Segment<2> &other) const {
     static_assert(Size == 2, "Only works for size == 2");
     double x1 = A_.x();
     double y1 = A_.y();