From 392cc287e6acea594bb68cb804375db2f291ef49 Mon Sep 17 00:00:00 2001
From: David Anson <david@dlaa.me>
Date: Fri, 16 Oct 2020 14:08:42 -0700
Subject: [PATCH] Handle implicit close fence in MD031/blanks-around-fences to
 avoid null-dereference.

---
 lib/md031.js                   | 11 ++++++-----
 test/lists-with-fenced-code.md |  7 +++++++
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/lib/md031.js b/lib/md031.js
index 5cfcfb4f..bfc4351c 100644
--- a/lib/md031.js
+++ b/lib/md031.js
@@ -21,7 +21,11 @@ module.exports = {
       if ((includeListItems || !inItem) &&
           ((onTopFence && !isBlankLine(lines[i - 1])) ||
            (onBottomFence && !isBlankLine(lines[i + 1])))) {
-        const [ , prefix ] = line.match(codeFencePrefixRe);
+        const [ , prefix ] = line.match(codeFencePrefixRe) || [];
+        const fixInfo = (prefix === undefined) ? null : {
+          "lineNumber": i + (onTopFence ? 1 : 2),
+          "insertText": `${prefix}\n`
+        };
         addErrorContext(
           onError,
           i + 1,
@@ -29,10 +33,7 @@ module.exports = {
           null,
           null,
           null,
-          {
-            "lineNumber": i + (onTopFence ? 1 : 2),
-            "insertText": `${prefix}\n`
-          });
+          fixInfo);
       }
     });
   }
diff --git a/test/lists-with-fenced-code.md b/test/lists-with-fenced-code.md
index 8e803be2..8c9aca86 100644
--- a/test/lists-with-fenced-code.md
+++ b/test/lists-with-fenced-code.md
@@ -57,3 +57,10 @@ Text
       fence {MD031:56} {MD031:58}
       ```
 1. Item
+
+Text
+
+1. Text
+   ```shell
+   fence {MD031:64} {MD031:65} {MD032:65} {MD040:66}
+  ```