JSON Parse Error

I hope this is the right forum

I’m using Piwik (0.4.3) on Drupal (6013).
The Drupl module (6.x-1.1) was installed and configured. When I try to get reports, with Firefox (3.0.1) I have the following error:

Open Flash Chart

JSON Parse Error [Syntax Error]
Error at character 0, line 1:
0:

Someone can help me?

Thanks