out_format() could be empty, too (when running outside knit())

Author: yihui

R/utils.R

@@ -383,7 +383,8 @@ is_latex_output = function() {
 #' @rdname output_type
 #' @export
 is_html_output = function(fmt = pandoc_to(), excludes = NULL) {
-  if (length(fmt) == 0) fmt = out_format()
+  fmt = fmt %n% out_format()
+  if (length(fmt) == 0) return(FALSE)
   if (grepl('^markdown', fmt)) fmt = 'markdown'
   if (fmt == 'epub3') fmt = 'epub'
   fmts = c('markdown', 'epub', 'epub2', 'html', 'html4', 'html5', 'revealjs', 's5', 'slideous', 'slidy', 'gfm')