[development] Custom validation function for Block configuration form?

Hi all, I've just created my own block module, and a configuration form for it. Now I just wonder whether it is possible to add my own validation function before Drupal save the block configuration? I tried to add the code like: $form[#validate][] = 'my_validation_function_name'; But it seems it doesn't help. So the question is whether Drupal support to do this during the configuration of block. If so, then how I can achieve this? Thanks in advance. travis