[TYPO3] Typo3 EXT develop degug question

[Rocky]

Hey Group,

I'm doing a TYPO3 EXT for visual debug purpose I add 
t3lib_div::debug($this->piVars); previous the content=' line of 
class.myext.php. I can see the debug table is there but there is an 
warning on the top of my browser says:
"Warning: Cannot modify header information - headers already sent by 
(output started at 
/home/lover/public_html/typo3_src-4.0.1/t3lib/class.t3lib_div.php:2779) 
in 
/home/lover/public_html/typo3_src-4.0.1/typo3/sysext/cms/tslib/class.tslib_fe.php 
on line 2826"

I do not think it is dabatase problem. Maybe I need to put the debug 
line somewhere else?

How can I get rid of this please?

Thanks,
Rocky