Custom Filter breaks saving CCK field data

I try to check the standard text field of a content type using the custom filter. Additionally the content type consists of some cck fields which should not be filtered. The problem: if the custom spam filter matches any defined spam-word and marks the node as spam it breaks saving the cck field values. Even required cck fields filled out before are empty. Because it's user generated content and there are empty required fields I could not publish this content after correcting/deleting the filtered word.