Error(s) in re-building vignettes:
  ...
--- re-building 'nlstimedist.Rmd' using rmarkdown
Quitting from lines 43-45 (nlstimedist.Rmd) 
Error: processing vignette 'nlstimedist.Rmd' failed with diagnostics:
cannot xtfrm data frames
--- failed re-building 'nlstimedist.Rmd'

SUMMARY: processing the following file failed:
  'nlstimedist.Rmd'

Error: Vignette re-building failed.
Execution halted
Error(s) in re-building vignettes:
  ...
--- re-building ‘nlstimedist.Rmd’ using rmarkdown
Quitting from lines 43-45 (nlstimedist.Rmd) 
Error: processing vignette 'nlstimedist.Rmd' failed with diagnostics:
cannot xtfrm data frames
--- failed re-building ‘nlstimedist.Rmd’

SUMMARY: processing the following file failed:
  ‘nlstimedist.Rmd’

Error: Vignette re-building failed.
Execution halted