diff --git a/jerry-core/ecma/builtin-objects/ecma-builtin-helpers.cpp b/jerry-core/ecma/builtin-objects/ecma-builtin-helpers.cpp
index 19ea0c718..258c3a8f0 100644
--- a/jerry-core/ecma/builtin-objects/ecma-builtin-helpers.cpp
+++ b/jerry-core/ecma/builtin-objects/ecma-builtin-helpers.cpp
@@ -594,7 +594,11 @@ ecma_builtin_helper_string_find_index (ecma_string_t *original_str_p, /**< index
 
       ecma_length_t index = start_pos;
 
-      lit_utf8_byte_t *original_str_curr_p = original_str_utf8_p + index;
+      lit_utf8_byte_t *original_str_curr_p = original_str_utf8_p;
+      for (ecma_length_t idx = 0; idx < index; idx++)
+      {
+        lit_utf8_incr (&original_str_curr_p);
+      }
 
       /* create utf8 string from search string */
       MEM_DEFINE_LOCAL_ARRAY (search_str_utf8_p,
diff --git a/tests/jerry/string-prototype-indexof.js b/tests/jerry/string-prototype-indexof.js
index 28a0eb8e2..ab8d08ccb 100644
--- a/tests/jerry/string-prototype-indexof.js
+++ b/tests/jerry/string-prototype-indexof.js
@@ -43,13 +43,14 @@ assert("w2\uFFA2A".indexOf("A") === 3);
 
 assert("w2\u1D306A".indexOf("A") === 4);
 
-// check surrogate pairs
 assert("\uD834\uDF06".indexOf("\uDF06") === 1);
 
 assert("\uD834\uDF06w2\u1D306D".indexOf("D") === 6);
 
 assert("\ud800\dc00".indexOf("\dc00") === 1);
 
+assert("\u8000\u0700\u8000\u8000A".indexOf("A", 3) === 4);
+
 // check prefix search
 assert("aaaabaaa".indexOf("aaaba") === 1);
 
diff --git a/tests/jerry/string-prototype-lastindexof.js b/tests/jerry/string-prototype-lastindexof.js
index 3f685f338..f25710fa5 100644
--- a/tests/jerry/string-prototype-lastindexof.js
+++ b/tests/jerry/string-prototype-lastindexof.js
@@ -39,6 +39,33 @@ assert("Hello welcome, welcome to the universe o.".lastIndexOf("o", 25) === 24);
 
 assert("Helloooo woooorld".lastIndexOf("oooo", 6) === 4);
 
+// check utf8 strings
+assert("\uFFA2".lastIndexOf("\uFFA2") === 0);
+
+assert("\uFFA2".lastIndexOf("A") === -1);
+
+assert("w2\uFFA2 A".lastIndexOf("A") === 4);
+
+assert("w2\u1D306A".indexOf("A") === 4);
+
+assert("\u0070A".lastIndexOf("A") === 1);
+
+assert("\u8000A".lastIndexOf("A") === 1);
+
+assert("\u0080\u0080\u0980\u1080A".lastIndexOf("A") === 4);
+
+assert("\u0080\u0980\u1080A\u0080\u0080\u0980\u1080".lastIndexOf("A", 4) === 3);
+
+assert("\u0080\u0080\u0980\u1080A\u0980AA\u0980A".lastIndexOf("A\u0980A") === 7);
+
+assert("\u0080\u0080\u0980\u1080A\u0980AA\u0980A".lastIndexOf("A\u0980A", 4) === 4);
+
+assert("\uD834\uDF06".lastIndexOf("\uDF06") === 1);
+
+assert("\uD834\uDF06w2\u1D306D\uDF06w2\u1D306D".lastIndexOf("D") === 12);
+
+assert("\ud800\dc00\ud800\dc00".lastIndexOf("\dc00") === 6);
+
 // check empty string
 assert(String.prototype.lastIndexOf.call(new String()) === -1);
 
@@ -75,7 +102,7 @@ assert("true".lastIndexOf(true, false) === 0);
 
 // check coercible - undefined
 try {
-  assert(String.prototype.lastIndexOf.call(undefined) === -1);
+  String.prototype.lastIndexOf.call(undefined);
   assert(false);
 } catch(e) {
   assert(e instanceof TypeError);
@@ -83,7 +110,7 @@ try {
 
 // check coercible - null
 try {
-  assert(String.prototype.lastIndexOf.call(null, 0) === -1);
+  String.prototype.lastIndexOf.call(null);
   assert(false);
 } catch (e) {
   assert(e instanceof TypeError);