r - 'object not found' error when knitting to HTML in RMarkdown -

i testing rmarkdown first time , running issue when trying knit html. example: attempt knit following html

# hello markdown  markdown **easy use** format writing reports. resembles naturally write every time compose email. in fact, may have used markdown *without realizing it*. these websites rely on markdown formatting  * [github](www.github.com) * [stackoverflow](www.stackoverflow.com) * [reddit](www.reddit.com) 

the above taken directly example on page: http://rmarkdown.rstudio.com/articles_intro.html

however, when use knit html button receive error:

error: object 'a' not found execution halted 

maybe simple question not seeing answer to, not know how interpret , correct error message, in markdown file calling sort of object 'a'. thank assistance!