diff --git a/json/src/ser.rs b/json/src/ser.rs
index 39a061a..bf7c7fb 100644
--- a/json/src/ser.rs
+++ b/json/src/ser.rs
@@ -956,10 +956,10 @@ pub trait Formatter {
     /// Writes a string fragment that doesn't need any escaping to the
     /// specified writer.
     #[inline]
-    fn write_string_fragment<W: ?Sized>(&mut self, writer: &mut W, fragment: &[u8]) -> Result<()>
+    fn write_string_fragment<W: ?Sized>(&mut self, writer: &mut W, fragment: &str) -> Result<()>
         where W: io::Write
     {
-        writer.write_all(fragment).map_err(From::from)
+        writer.write_all(fragment.as_bytes()).map_err(From::from)
     }
 
     /// Writes a character escape code to the specified writer.
@@ -1248,7 +1248,7 @@ fn format_escaped_str_contents<W: ?Sized, F: ?Sized>(writer: &mut W,
         }
 
         if start < i {
-            try!(formatter.write_string_fragment(writer, &bytes[start..i]));
+            try!(formatter.write_string_fragment(writer, &value[start..i]));
         }
 
         let char_escape = CharEscape::from_escape_table(escape, byte);
@@ -1258,7 +1258,7 @@ fn format_escaped_str_contents<W: ?Sized, F: ?Sized>(writer: &mut W,
     }
 
     if start != bytes.len() {
-        try!(formatter.write_string_fragment(writer, &bytes[start..]));
+        try!(formatter.write_string_fragment(writer, &value[start..]));
     }
 
     Ok(())