index out of range panic in isReference

The below program panics:

package main

import "github.com/russross/blackfriday"

func main() {
    renderer := blackfriday.HtmlRenderer(blackfriday.HTML_USE_XHTML, "", "")
    blackfriday.Markdown([]byte("   ["), renderer,
        blackfriday.EXTENSION_TABLES|
            blackfriday.EXTENSION_AUTOLINK|
            blackfriday.EXTENSION_FOOTNOTES|
            blackfriday.EXTENSION_TITLEBLOCK|
            blackfriday.EXTENSION_HEADER_IDS|
            blackfriday.EXTENSION_FENCED_CODE|
            blackfriday.EXTENSION_SPACE_HEADERS|
            blackfriday.EXTENSION_STRIKETHROUGH|
            blackfriday.EXTENSION_TAB_SIZE_EIGHT|
            blackfriday.EXTENSION_AUTO_HEADER_IDS|
            blackfriday.EXTENSION_LAX_HTML_BLOCKS|
            blackfriday.EXTENSION_HARD_LINE_BREAK|
            blackfriday.EXTENSION_NO_INTRA_EMPHASIS|
            blackfriday.EXTENSION_BACKSLASH_LINE_BREAK|
            blackfriday.EXTENSION_NO_EMPTY_LINE_BEFORE_BLOCK,
    )
}