cannot convert xlsm to xlsx using ChangeDocumentType

 i tried to use openxml's changedocumenttype to convert xlsm to xlsx file,
 but when it was done, i cannot open it , the office showed me the xlsx file was broken.

 who can tell me  why was that? this is the code:

    //convert xlsm to xlsx
    using (SpreadsheetDocument myDoc = SpreadsheetDocument.Open(xlsmPath, true))
    {
         myDoc.ChangeDocumentType(SpreadsheetDocumentType.Workbook);
    }