2 files changed, 11 insertions, 8 deletions
diff --git a/mrbgems/mruby-string-ext/mrblib/string.rb b/mrbgems/mruby-string-ext/mrblib/string.rb
index 3dd5561c8..fc734b951 100644
--- a/mrbgems/mruby-string-ext/mrblib/string.rb
+++ b/mrbgems/mruby-string-ext/mrblib/string.rb
@@ -254,14 +254,13 @@ class String
   #     "abcd".insert(-1, 'X')   #=> "abcdX"
   #
   def insert(idx, str)
-    pos = idx.to_i
-    pos += self.size + 1 if pos < 0
-
-    raise IndexError, "index #{idx.to_i} out of string" if pos < 0 || pos > self.size
-
-    return self + str if pos == -1
-    return str + self if pos == 0
-    return self[0..pos - 1] + str + self[pos..-1]
+    if idx == -1
+      return self << str
+    elsif idx < 0
+      idx += 1
+    end
+    self[idx, 0] = str
+    self
   end
 
   ##
diff --git a/mrbgems/mruby-string-ext/test/string.rb b/mrbgems/mruby-string-ext/test/string.rb
index f68fbcb28..aa3e8a192 100644
--- a/mrbgems/mruby-string-ext/test/string.rb
+++ b/mrbgems/mruby-string-ext/test/string.rb
@@ -406,6 +406,10 @@ assert('String#insert') do
   assert_equal "abcdX", "abcd".insert(-1, 'X')
   assert_raise(IndexError) { "abcd".insert(5, 'X') }
   assert_raise(IndexError) { "abcd".insert(-6, 'X') }
+
+  a = "abcd"
+  a.insert(0, 'X')
+  assert_equal "Xabcd", a
 end
 
 assert('String#prepend') do