Error(s) in re-building vignettes:
  ...
--- re-building 'ndvtest.Rmd' using rmarkdown
Quitting from lines 641-648 (ndvtest.Rmd) 
Error: processing vignette 'ndvtest.Rmd' failed with diagnostics:
input string 2 is invalid
--- failed re-building 'ndvtest.Rmd'

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

Error: Vignette re-building failed.
Execution halted