Convert wmf to mathml crash after running many times

Use mathtype sdk 6.9 to convert many wmf to mathml file, after converting several hundreds of wmf files successfully, it crashed (unhandled exception) in below position of MathTypeSDK.cs(MTXFormSetTranslatorMgn function)

retCode = MTXFormSetTranslator(options, transName);

the exception details are:

System.AccessViolationException
  HResult=0x80004003
  Message=Attempted to read or write protected memory. This is often an indication that other memory is corrupt
  Source=.....
  StackTrace:
.....

My convertion code is as below:

ConvertEquation ce = new ConvertEquation();
return ce.Convert(new EquationInputFileWMF(wmfFile),
    new EquationOutputFileText(outputML, "MathJax-MathML.tdl"));