[Nico]

Remove the equal sign in this line:

Code:

for(i=0;i<=aantal;i++)

... it makes the loop run one time too much, hence you get the error.

EDIT:

And this line:

Code:

window.onload = pi();

.... should be:

Code:

window.onload = pi;

... with the parenthesis, you're calling the function. And if it's being called before the browser has parsed the full page, it's possible that you get this error too.