diff --git a/src/bytes/buffer.go b/src/bytes/buffer.go
index aff2db5084..190c468162 100644
--- a/src/bytes/buffer.go
+++ b/src/bytes/buffer.go
@@ -385,13 +385,15 @@ func (b *Buffer) UnreadRune() error {
 	return nil
 }
 
+var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
+
 // UnreadByte unreads the last byte returned by the most recent successful
 // read operation that read at least one byte. If a write has happened since
 // the last read, if the last read returned an error, or if the read read zero
 // bytes, UnreadByte returns an error.
 func (b *Buffer) UnreadByte() error {
 	if b.lastRead == opInvalid {
-		return errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
+		return errUnreadByte
 	}
 	b.lastRead = opInvalid
 	if b.off > 0 {