Problem with performance counter??

hi,

i use a performance counter and try print execution time of my design but it print the message bellow:( time (sec) is 00??

Total Time: 0 seconds (0 clock-cycles)

+---------------+-----+-----------+---------------+-----------+

| Section | % | Time (sec)| Time (clocks)|Occurrences|

+---------------+-----+-----------+---------------+-----------+

|z1 | NaN| 0.00000| 0| 0|

+---------------+-----+-----------+---------------+-----------+

thank for all members