Why is Katrina Kaif afraid of Shahrukh Khan?

The gorgeous Katrina Kaif, one of the few actresses who has worked with the three ruling Khan’s of Bollywood has revealed that she was quite comfortable working with Salman and Aamir but scared while working with Shahrukh Khan.

According to sources, Katrina Kaif when asked about as to which Khan she is most comfortable with replied, “I would have to say Salman. He’s seen me right from the beginning; so I wasn’t conscious or worried about messing up a line. I have known him for so long that I know he won’t judge me. With Aamir initially was nervous but later had a lot of fun while shooting for Dhoom 3.”

But with SRK, “Something about Shahrukh made me nervous… I don’t know why. When we weren’t shooting, everything was fine. We’d be chatting and having fun, but as soon as Yashji (Chopra) would say ‘action’, I’d freeze.”
Surprisingly Shahrukh is known to make his heroines quite comfortable, so what happened with Kat?