() doesn't implement std::fmt::Display

[intellild]

rustc 1.26.0 (a77568041 2018-05-07)

error[E0277]: `()` doesn't implement `std::fmt::Display`
  --> /Users/intellild/.cargo/registry/src/mirrors.ustc.edu.cn-61ef6e0cd06fb9b8/sockjs-0.3.0/src/transports/xhrsend.rs:89:49
   |
89 |                                             Err(error::ErrorNotFound(())),
   |                                                 ^^^^^^^^^^^^^^^^^^^^ `()` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string
   |
   = help: the trait `std::fmt::Display` is not implemented for `()`
   = note: required by `actix_web::error::ErrorNotFound`

error[E0277]: `()` doesn't implement `std::fmt::Display`
   --> /Users/intellild/.cargo/registry/src/mirrors.ustc.edu.cn-61ef6e0cd06fb9b8/sockjs-0.3.0/src/transports/jsonp.rs:271:49
    |
271 |                                             Err(error::ErrorNotFound(())),
    |                                                 ^^^^^^^^^^^^^^^^^^^^ `()` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string
    |
    = help: the trait `std::fmt::Display` is not implemented for `()`
    = note: required by `actix_web::error::ErrorNotFound`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.
error: Could not compile `sockjs`.