FCKeditorAPI is null or not an object errors

Hi all
I'm using the fckeditor api to insert some html to the editor
i'm working on an aspx web form
when i exceute the function i receive the following two errors

FCKeditorAPI is null or not an object
and
FCKTools is undefined
the function is as follows

function fckTest()
{
var x=FCKeditorAPI.GetInstance('editorName') ;
x.InsertHtml('<strong>Hello</strong>');
}

please tell me what is the reason for that
thanks