get_citation_history(id) gives error for some id's about scholar HOT 7 CLOSED

get_citation_history <- function(id) {

    ## Ensure only one ID
    id <- tidy_id(id)

    ## Read the page and parse the key data
    url_template <- "http://scholar.google.com/citations?hl=en&user=%s&pagesize=100&view_op=list_works"
    url <- sprintf(url_template, id)

    ## A better way would actually be to read out the plot of citations
    page <- GET(url, handle=getOption("scholar_handle")) %>% read_html()
    years <- page %>% html_nodes(xpath="//*/span[@class='gsc_g_t']") %>%
        html_text() %>% as.numeric()
    vals <- page %>% html_nodes(xpath="//*/span[@class='gsc_g_al']") %>%
        html_text() %>% as.numeric()
    if(length(years)>length(vals)){
      # Some years don't have citations. 
      # We need to match the citation counts and years
      # <a href="javascript:void(0)" class="gsc_g_a" style="left:8px;height:5px;z-index:9">\n  <span class="gsc_g_al">2</span>\n</a>
      style_tags=page %>% html_nodes(css = '.gsc_g_a') %>% 
        html_attr('style')
      # these z indices seem to be the indices starting with the last year
      zindices=as.integer(stringr::str_match(style_tags, 'z-index:([0-9]+)')[,2])
      # empty vector of 0s
      allvals=integer(length=length(years))
      # fill in 
      allvals[zindices]=rev(vals)
      # and then reverse
      vals=rev(allvals)
    }
    df <- data.frame(year=years, cites=vals)

    return(df)
}