Change the declaration part of the counter variable into on the loop.

lib/plugins/tag/img.js

@@ -35,27 +35,26 @@ module.exports = ctx => {
   return function imgTag(args, content) {
     const classes = [];
     let src;
-    let i = 0;
 
     // Find image URL and class name
-    for (const len = args.length; i < len; i++) {
+    for (let i = 0, len = args.length; i < len; i++) {
       const item = args[i];
 
       if (rUrl.test(item) || item[0] === '/') {
         src = makeUrl(item);
+
+        // Delete image URL and class name from arguments
+        args = args.slice(i + 1);
         break;
       } else {
         classes.push(item);
       }
     }
 
-    // Delete image URL and class name from arguments
-    args = args.slice(i + 1);
-
     let width, height, title, alt;
 
     // Find image width and height
-    if (args.length) {
+    if (args && args.length) {
       if (!/\D+/.test(args[0])) {
         width = args.shift();