Script error

I want to load a value of other veld in the editor.

i tried this:

oFCKeditor.Value=document.MailMessage.textfieldHTLM.value ;

but i get A mismacht type javascript error when i sumbit the form.
the java script error occures in the fckeditorcode_ie_1.js file line 17: char: 6037

Hwo has already slove this error?? and how