diff --git a/mrblib/string.rb b/mrblib/string.rb
index 47fe59026..51be9b7dd 100644
--- a/mrblib/string.rb
+++ b/mrblib/string.rb
@@ -27,8 +27,8 @@ class String
     end
     start = 0
     string = dup
-    self_len = length
-    sep_len = separator.length
+    self_len = self.bytesize
+    sep_len = separator.bytesize
 
     while (pointer = string.byteindex(separator, start))
       pointer += sep_len