diff --git a/lib/util.js b/lib/util.js
index 1f810af1..7d106b3c 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -74,30 +74,19 @@ function isProtoString(s) {
     return false;
   }
 
-  /* eslint-disable no-multi-spaces */
-  if (
-    s.charCodeAt(length - 1) !== 95 /* '_' */ ||
-    s.charCodeAt(length - 2) !== 95 /* '_' */ ||
-    s.charCodeAt(length - 3) !== 111 /* 'o' */ ||
-    s.charCodeAt(length - 4) !== 116 /* 't' */ ||
-    s.charCodeAt(length - 5) !== 111 /* 'o' */ ||
-    s.charCodeAt(length - 6) !== 114 /* 'r' */ ||
-    s.charCodeAt(length - 7) !== 112 /* 'p' */ ||
-    s.charCodeAt(length - 8) !== 95 /* '_' */ ||
-    s.charCodeAt(length - 9) !== 95 /* '_' */
-  ) {
+  if (s.substring(length - 9) !== "__proto__") {
     return false;
   }
-  /* eslint-enable no-multi-spaces */
 
   for (let i = length - 10; i >= 0; i--) {
-    if (s.charCodeAt(i) !== 36 /* '$' */) {
+    if (s[i] !== "$") {
       return false;
     }
   }
 
   return true;
 }
+exports.isProtoString = isProtoString;
 
 function strcmp(aStr1, aStr2) {
   if (aStr1 === aStr2) {
diff --git a/test/test-util.js b/test/test-util.js
index 9404f061..6188465d 100644
--- a/test/test-util.js
+++ b/test/test-util.js
@@ -300,3 +300,20 @@ exports["test computeSourceURL"] = function(assert) {
     "http://example.com/dir/test.js"
   );
 };
+
+exports["test isProtoString"] = async function(assert) {
+  assert.equal(libUtil.isProtoString("a.js"), false);
+  assert.equal(libUtil.isProtoString("test$__proto__"), false);
+  assert.equal(libUtil.isProtoString("$$$$$$$$$$$$$dfgdfgdf__proto__"), false);
+  assert.equal(libUtil.isProtoString("dddddd$$$$$$$$$$$$$__proto__"), false);
+  assert.equal(
+    libUtil.isProtoString("$$$$$$$$$$$$$__proto__$$$$$$$$$$$$$__proto__"),
+    false
+  );
+  assert.equal(libUtil.isProtoString("blabla"), false);
+  assert.equal(libUtil.isProtoString(""), false);
+
+  assert.equal(libUtil.isProtoString("__proto__"), true);
+  assert.equal(libUtil.isProtoString("$__proto__"), true);
+  assert.equal(libUtil.isProtoString("$$$$$$$$$$$$$__proto__"), true);
+};