diff --git a/js/src/jsstr.cpp b/js/src/jsstr.cpp
index afe2b8bd4947..48e955638f0a 100644
--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -1700,26 +1700,31 @@ LastIndexOfImpl(const TextChar* text, size_t textLen, const PatChar* pat, size_t
     return -1;
 }
 
+// ES2017 draft rev 6859bb9ccaea9c6ede81d71e5320e3833b92cb3e
+// 21.1.3.9 String.prototype.lastIndexOf ( searchString [ , position ] )
 bool
 js::str_lastIndexOf(JSContext* cx, unsigned argc, Value* vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
-    RootedString textstr(cx, ToStringForStringFunction(cx, args.thisv()));
-    if (!textstr)
+
+    // Steps 1-2.
+    RootedString str(cx, ToStringForStringFunction(cx, args.thisv()));
+    if (!str)
         return false;
 
-    RootedLinearString pat(cx, ArgToRootedString(cx, args, 0));
-    if (!pat)
+    // Step 3.
+    RootedLinearString searchStr(cx, ArgToRootedString(cx, args, 0));
+    if (!searchStr)
         return false;
 
-    size_t textLen = textstr->length();
-    size_t patLen = pat->length();
-    int start = textLen - patLen; // Start searching here
-    if (start < 0) {
-        args.rval().setInt32(-1);
-        return true;
-    }
+    // Step 6.
+    size_t len = str->length();
 
+    // Step 8.
+    size_t searchLen = searchStr->length();
+
+    // Steps 4-5, 7.
+    int start = len - searchLen; // Start searching here
     if (args.hasDefined(1)) {
         if (args[1].isInt32()) {
             int i = args[1].toInt32();
@@ -1741,29 +1746,36 @@ js::str_lastIndexOf(JSContext* cx, unsigned argc, Value* vp)
         }
     }
 
-    if (patLen == 0) {
-        args.rval().setInt32(start);
+    if (searchLen > len) {
+        args.rval().setInt32(-1);
         return true;
     }
 
-    JSLinearString* text = textstr->ensureLinear(cx);
+    if (searchLen == 0) {
+        args.rval().setInt32(start);
+        return true;
+    }
+    MOZ_ASSERT(0 <= start && size_t(start) < len);
+
+    JSLinearString* text = str->ensureLinear(cx);
     if (!text)
         return false;
 
+    // Step 9.
     int32_t res;
     AutoCheckCannotGC nogc;
     if (text->hasLatin1Chars()) {
         const Latin1Char* textChars = text->latin1Chars(nogc);
-        if (pat->hasLatin1Chars())
-            res = LastIndexOfImpl(textChars, textLen, pat->latin1Chars(nogc), patLen, start);
+        if (searchStr->hasLatin1Chars())
+            res = LastIndexOfImpl(textChars, len, searchStr->latin1Chars(nogc), searchLen, start);
         else
-            res = LastIndexOfImpl(textChars, textLen, pat->twoByteChars(nogc), patLen, start);
+            res = LastIndexOfImpl(textChars, len, searchStr->twoByteChars(nogc), searchLen, start);
     } else {
         const char16_t* textChars = text->twoByteChars(nogc);
-        if (pat->hasLatin1Chars())
-            res = LastIndexOfImpl(textChars, textLen, pat->latin1Chars(nogc), patLen, start);
+        if (searchStr->hasLatin1Chars())
+            res = LastIndexOfImpl(textChars, len, searchStr->latin1Chars(nogc), searchLen, start);
         else
-            res = LastIndexOfImpl(textChars, textLen, pat->twoByteChars(nogc), patLen, start);
+            res = LastIndexOfImpl(textChars, len, searchStr->twoByteChars(nogc), searchLen, start);
     }
 
     args.rval().setInt32(res);
diff --git a/js/src/tests/ecma/String/lastIndexOf-ToNumber-when-searchStr-larger-than-string.js b/js/src/tests/ecma/String/lastIndexOf-ToNumber-when-searchStr-larger-than-string.js
new file mode 100644
index 000000000000..a4c3c316e07b
--- /dev/null
+++ b/js/src/tests/ecma/String/lastIndexOf-ToNumber-when-searchStr-larger-than-string.js
@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+var str = "a";
+var searchStr = "abc";
+
+var position = new Object();
+var valueOfCalled = false;
+function positionValueOf() {
+    assertEq(valueOfCalled, false);
+    valueOfCalled = true;
+    return 0;
+}
+position.valueOf = positionValueOf;
+
+var result = str.lastIndexOf(searchStr, position);
+assertEq(result, -1);
+assertEq(valueOfCalled, true);
+
+if (typeof reportCompare == "function")
+    reportCompare(0, 0);