Comparator and minecarts

Comparator sholud light, right?
I expected the comparator to light up while testing for items in the minecart
with chest.

But no!
When I placed the comparator it didn't light up
(Yes there were items in the minecart with chest)

Steps to Reproduce:
1.Place a rail on the ground
2.Place the minecart with chest
3.Put something in the chest
4.Place the comparator to test for items in minecart's chest
5.Look at comparator