invalid summary problem while debugging in Xcode

I have been trying to debug on Xcode and am facing the following problem:

Most of the times when I hover my mouse point开发者_如何学编程er over a variable to see its value I get invalid summary. However, in some cases i have a NSLog statement after that which prints the value of the same variable quite ok !

Can anyone kindly help me out with this weird problem ? Thanks.