Oracle Message ORA-26748 - The one-to-one transformation function string encountered the following error: string

ERRORThe one-to-one transformation function string encountered the following error: string
CAUSEThe specified transformation function encountered an error.
ACTIONEnsure that the function does not return an LCR that has a different type from the LCR which was passed to the function. Also ensure that the function does not return NULL. For DDL transformation functions, creating and returning a new DDL LCR is not allowed.