After "Salting the Earth" mission in Saints Row 2, I want to do stronghold in Nob Hill, but i cant find where ?

I have just finished the "Salting the Earth" mission in Saints Row 2, and I am trying to to the Ultor Corp. stronghold in the Nob Hill area, but when I go to where it says I need to go to on the map, there is nothing there for me to stand in to start the stronghold mission. What do I do?