phpBB2 to Drupal 5.1

I am using Drupal 5.1 and i have installed phpBB2. I have followed the steps but now i got following error in form.inc.

Fatal error: Cannot use string offset as an array in C:\wamp\www\drupal\includes\form.inc on line 676

Above error will generate when i an trying to click on Configure link under Administrator -> Site Setting -> Blocks.

I have make one database for both Drupal and phpBB2