diff --git a/lib/index.umd.js b/lib/index.umd.js
index 0732195..b4cea17 100644
--- a/lib/index.umd.js
+++ b/lib/index.umd.js
@@ -94,6 +94,10 @@
         var walk = 0;
 
         while (level > 0 || walk === 0) {
+          if (walk >= src.length) {
+            return null;
+          }
+
           if (src[walk] + src[walk + 1] === '$[') {
             level++;
             walk++;
diff --git a/package.json b/package.json
index eee9682..c7566f4 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "marked-mfm",
-  "version": "0.3.1",
+  "version": "0.3.2",
   "description": "Marked.js extension for Misskey-flavored Markdown",
   "main": "./src/index.js",
   "browser": "./lib/index.umd.js",
diff --git a/src/index.js b/src/index.js
index 6c07a70..c899aa3 100644
--- a/src/index.js
+++ b/src/index.js
@@ -10,6 +10,9 @@ export default {
         let level = 0
         let walk = 0
         while (level > 0 || walk === 0) {
+          if (walk >= src.length) {
+            return null
+          }
           if (src[walk] + src[walk + 1] === '$[') {
             level++
             walk++