diff --git a/json/src/ser.rs b/json/src/ser.rs
index c81bff269d..f41ef7e348 100644
--- a/json/src/ser.rs
+++ b/json/src/ser.rs
@@ -950,7 +950,7 @@ fn fmt_f64_or_null<W>(wr: &mut W, value: f64) -> Result<()>
 
 /// Encode the specified struct into a json `[u8]` writer.
 #[inline]
-pub fn to_writer<W, T>(writer: &mut W, value: &T) -> Result<()>
+pub fn to_writer<W: ?Sized, T>(writer: &mut W, value: &T) -> Result<()>
     where W: io::Write,
           T: ser::Serialize,
 {
@@ -961,7 +961,7 @@ pub fn to_writer<W, T>(writer: &mut W, value: &T) -> Result<()>
 
 /// Encode the specified struct into a json `[u8]` writer.
 #[inline]
-pub fn to_writer_pretty<W, T>(writer: &mut W, value: &T) -> Result<()>
+pub fn to_writer_pretty<W: ?Sized, T>(writer: &mut W, value: &T) -> Result<()>
     where W: io::Write,
           T: ser::Serialize,
 {