operate by instruction doesn't work while debugging the code

Hi,
I am using QT 4.7.3. While debugging the code, it goes in disassembler. Pressing "Operate by instruction" does't work. I am unable to de-select "Operate by instruction". It still remains in disassembler. I have GDB debugger. How to switch off disassembler and view the line where I have set the breakpoint.
Thanks

Make sure you have a debug build of whatever you are trying to step into using the debugger.

Yes it is a debug build. Still it's not working :(

Please post the backtrace.