From 9dec7e9e8bb854ef86ee7fa5485b9fa1192136b1 Mon Sep 17 00:00:00 2001
From: yuin <yuin@inforno.net>
Date: Wed, 20 Nov 2019 01:10:18 +0900
Subject: [PATCH] Fixes #31

---
 _test/options.txt     |  9 +++++++++
 parser/atx_heading.go | 10 ++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/_test/options.txt b/_test/options.txt
index df4351a..e24e360 100644
--- a/_test/options.txt
+++ b/_test/options.txt
@@ -28,3 +28,12 @@
 <h2 id="id_7" attr7="value &quot;7">Title7</h2>
 <h2 id="id" class="className class1 class2" attrName="attrValue">Title8</h2>
 //= = = = = = = = = = = = = = = = = = = = = = = =//
+
+2
+//- - - - - - - - -//
+#
+# FOO
+//- - - - - - - - -//
+<h1 id="heading"></h1>
+<h1 id="foo">FOO</h1>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
diff --git a/parser/atx_heading.go b/parser/atx_heading.go
index dee28af..04278ab 100644
--- a/parser/atx_heading.go
+++ b/parser/atx_heading.go
@@ -186,15 +186,21 @@ func (b *atxHeadingParser) CanAcceptIndentedLine() bool {
 var attrAutoHeadingIDPrefix = []byte("heading")
 
 func generateAutoHeadingID(node *ast.Heading, reader text.Reader, pc Context) {
+	var line []byte
 	lastIndex := node.Lines().Len() - 1
-	lastLine := node.Lines().At(lastIndex)
-	line := lastLine.Value(reader.Source())
+	if lastIndex > -1 {
+		lastLine := node.Lines().At(lastIndex)
+		line = lastLine.Value(reader.Source())
+	}
 	headingID := pc.IDs().Generate(line, attrAutoHeadingIDPrefix)
 	node.SetAttribute(attrNameID, headingID)
 }
 
 func parseLastLineAttributes(node ast.Node, reader text.Reader, pc Context) {
 	lastIndex := node.Lines().Len() - 1
+	if lastIndex < 0 { // empty headings
+		return
+	}
 	lastLine := node.Lines().At(lastIndex)
 	line := lastLine.Value(reader.Source())
 	lr := text.NewReader(line)