diff --git a/js/src/builtin/String.cpp b/js/src/builtin/String.cpp
index 83bb72716647..16e92b554c91 100644
--- a/js/src/builtin/String.cpp
+++ b/js/src/builtin/String.cpp
@@ -2652,10 +2652,14 @@ bool js::StringLastIndexOf(JSContext* cx, HandleString string,
     return true;
   }
 
+  MOZ_ASSERT(len >= searchLen);
+  size_t start = len - searchLen;
+
   if (searchLen == 0) {
-    *result = 0;
+    *result = start;
     return true;
   }
+  MOZ_ASSERT(start < len);
 
   JSLinearString* text = string->ensureLinear(cx);
   if (!text) {
@@ -2667,9 +2671,6 @@ bool js::StringLastIndexOf(JSContext* cx, HandleString string,
     return false;
   }
 
-  MOZ_ASSERT(len >= searchLen);
-  size_t start = len - searchLen;
-
   *result = LastIndexOf(text, searchStr, start);
   return true;
 }
diff --git a/js/src/jit-test/tests/cacheir/string-lastIndexOf.js b/js/src/jit-test/tests/cacheir/string-lastIndexOf.js
new file mode 100644
index 000000000000..f281d1eac123
--- /dev/null
+++ b/js/src/jit-test/tests/cacheir/string-lastIndexOf.js
@@ -0,0 +1,102 @@
+function testEmpty() {
+  var strings = [
+    "",
+    "a",
+    "ab",
+  ];
+
+  for (var i = 0; i < 200; ++i) {
+    var str = strings[i % strings.length];
+    assertEq(str.lastIndexOf(""), str.length);
+  }
+}
+testEmpty();
+
+function testSingle() {
+  var strings = [
+    "",
+
+    "a",
+    "b",
+
+    "aa",
+    "ab",
+    "ba",
+    "bb",
+  ];
+
+  var searchStrings = [
+    "a",
+    "b",
+  ];
+
+  for (var i = 0; i < 200; ++i) {
+    var str = strings[i % strings.length];
+    var searchString = searchStrings[i % searchStrings.length];
+
+    var j = str.length;
+    while (--j >= 0 && str[j] !== searchString);
+
+    assertEq(str.lastIndexOf(searchString), j);
+  }
+}
+testSingle();
+
+function testDouble() {
+  var strings = [
+    "",
+
+    "a",
+    "b",
+
+    "aa",
+    "ab",
+    "ba",
+    "bb",
+
+    "aaa",
+    "aab",
+    "aba",
+    "abb",
+    "baa",
+    "bab",
+    "bba",
+    "bbb",
+
+    "aaaa",
+    "aaab",
+    "aaba",
+    "aabb",
+    "abaa",
+    "abab",
+    "abba",
+    "abbb",
+
+    "baaa",
+    "baab",
+    "baba",
+    "babb",
+    "bbaa",
+    "bbab",
+    "bbba",
+    "bbbb",
+  ];
+
+  var searchStrings = [
+    "aa",
+    "ab",
+    "ba",
+    "bb",
+  ];
+
+  for (var i = 0; i < 200; ++i) {
+    var str = strings[i % strings.length];
+    var searchString = searchStrings[i % searchStrings.length];
+
+    var j = str.length;
+    while (--j >= 0 && (str[j] !== searchString[0] || str[j + 1] !== searchString[1]));
+
+    assertEq(str.lastIndexOf(searchString), j);
+  }
+}
+testDouble();