diff --git a/openssl/src/ssl/callbacks.rs b/openssl/src/ssl/callbacks.rs
index ca257e13..0531274c 100644
--- a/openssl/src/ssl/callbacks.rs
+++ b/openssl/src/ssl/callbacks.rs
@@ -2,8 +2,6 @@ use ffi;
 use libc::{c_char, c_int, c_uchar, c_uint, c_void};
 #[cfg(all(feature = "v111", ossl111))]
 use libc::size_t;
-#[cfg(all(feature = "v111", ossl111))]
-use std::borrow::Cow;
 use std::ffi::CStr;
 use std::ptr;
 use std::slice;
@@ -426,17 +424,18 @@ where
 }
 
 #[cfg(all(feature = "v111", ossl111))]
-pub struct CustomExtAddState(Option<Cow<'static, [u8]>>);
+pub struct CustomExtAddState<T>(Option<T>);
 
 #[cfg(all(feature = "v111", ossl111))]
-pub extern "C" fn raw_custom_ext_add<F>(ssl: *mut ffi::SSL, _: c_uint,
-                                        context: c_uint,
-                                        out: *mut *const c_uchar,
-                                        outlen: *mut size_t, x: *mut ffi::X509,
-                                        chainidx: size_t, al: *mut c_int,
-                                        _: *mut c_void)
-                                        -> c_int
-    where F: Fn(&mut SslRef, ExtensionContext, Option<(usize, &X509Ref)>) -> Result<Option<Cow<'static, [u8]>>, SslAlert> + 'static
+pub extern "C" fn raw_custom_ext_add<F, T>(ssl: *mut ffi::SSL, _: c_uint,
+                                           context: c_uint,
+                                           out: *mut *const c_uchar,
+                                           outlen: *mut size_t, x: *mut ffi::X509,
+                                           chainidx: size_t, al: *mut c_int,
+                                           _: *mut c_void)
+                                           -> c_int
+    where F: Fn(&mut SslRef, ExtensionContext, Option<(usize, &X509Ref)>) -> Result<Option<T>, SslAlert> + 'static,
+          T: AsRef<[u8]> + 'static,
 {
     unsafe {
         let ssl_ctx = ffi::SSL_get_SSL_CTX(ssl as *const _);
@@ -448,13 +447,13 @@ pub extern "C" fn raw_custom_ext_add<F>(ssl: *mut ffi::SSL, _: c_uint,
         match (callback)(ssl, ectx, cert) {
             Ok(None) => 0,
             Ok(Some(buf)) => {
-                *outlen = buf.len() as size_t;
-                *out = buf.as_ptr();
+                *outlen = buf.as_ref().len() as size_t;
+                *out = buf.as_ref().as_ptr();
 
-                let idx = get_ssl_callback_idx::<CustomExtAddState>();
+                let idx = get_ssl_callback_idx::<CustomExtAddState<T>>();
                 let ptr = ffi::SSL_get_ex_data(ssl.as_ptr(), idx);
                 if ptr.is_null() {
-                    let x = Box::into_raw(Box::<CustomExtAddState>::new(CustomExtAddState(Some(buf)))) as *mut c_void;
+                    let x = Box::into_raw(Box::<CustomExtAddState<T>>::new(CustomExtAddState(Some(buf)))) as *mut c_void;
                     ffi::SSL_set_ex_data(ssl.as_ptr(), idx, x);
                 } else {
                     *(ptr as *mut _) = CustomExtAddState(Some(buf))
@@ -470,14 +469,15 @@ pub extern "C" fn raw_custom_ext_add<F>(ssl: *mut ffi::SSL, _: c_uint,
 }
 
 #[cfg(all(feature = "v111", ossl111))]
-pub extern "C" fn raw_custom_ext_free(ssl: *mut ffi::SSL, _: c_uint,
-                                      _: c_uint,
-                                      _: *mut *const c_uchar,
-                                      _: *mut c_void)
+pub extern "C" fn raw_custom_ext_free<T>(ssl: *mut ffi::SSL, _: c_uint,
+                                         _: c_uint,
+                                         _: *mut *const c_uchar,
+                                         _: *mut c_void)
+    where T: 'static
 {
     unsafe {
-        let state = ffi::SSL_get_ex_data(ssl, get_ssl_callback_idx::<CustomExtAddState>());
-        let state = &mut (*(state as *mut CustomExtAddState)).0;
+        let state = ffi::SSL_get_ex_data(ssl, get_ssl_callback_idx::<CustomExtAddState<T>>());
+        let state = &mut (*(state as *mut CustomExtAddState<T>)).0;
         state.take();
     }
 }
diff --git a/openssl/src/ssl/mod.rs b/openssl/src/ssl/mod.rs
index 402fa1ce..6a140c14 100644
--- a/openssl/src/ssl/mod.rs
+++ b/openssl/src/ssl/mod.rs
@@ -61,8 +61,6 @@ use ffi;
 use foreign_types::{ForeignType, ForeignTypeRef, Opaque};
 use libc::{c_char, c_int, c_long, c_uchar, c_uint, c_ulong, c_void};
 use std::any::TypeId;
-#[cfg(all(feature = "v111", ossl111))]
-use std::borrow::Cow;
 use std::cmp;
 use std::collections::HashMap;
 use std::ffi::{CStr, CString};
@@ -1511,10 +1509,12 @@ impl SslContextBuilder {
     ///
     /// [`SSL_CTX_add_custom_ext`]: https://www.openssl.org/docs/manmaster/man3/SSL_CTX_add_custom_ext.html
     #[cfg(all(feature = "v111", ossl111))]
-    pub fn add_custom_ext<AddFn, ParseFn>(&mut self, ext_type: u16, context: ExtensionContext, add_cb: AddFn, parse_cb: ParseFn)
-                                          -> Result<(), ErrorStack>
+    pub fn add_custom_ext<AddFn, ParseFn, T>(
+        &mut self, ext_type: u16, context: ExtensionContext, add_cb: AddFn, parse_cb: ParseFn
+    ) -> Result<(), ErrorStack>
         where AddFn: Fn(&mut SslRef, ExtensionContext, Option<(usize, &X509Ref)>)
-                        -> Result<Option<Cow<'static, [u8]>>, SslAlert> + 'static + Sync + Send,
+                        -> Result<Option<T>, SslAlert> + 'static + Sync + Send,
+              T: AsRef<[u8]> + 'static,
               ParseFn: Fn(&mut SslRef, ExtensionContext, &[u8], Option<(usize, &X509Ref)>)
                           -> Result<(), SslAlert> + 'static + Sync + Send,
     {
@@ -1534,8 +1534,8 @@ impl SslContextBuilder {
             );
 
             ffi::SSL_CTX_add_custom_ext(self.as_ptr(), ext_type as c_uint, context.bits(),
-                                        Some(raw_custom_ext_add::<AddFn>),
-                                        Some(raw_custom_ext_free),
+                                        Some(raw_custom_ext_add::<AddFn, T>),
+                                        Some(raw_custom_ext_free::<T>),
                                         ptr::null_mut(),
                                         Some(raw_custom_ext_parse::<ParseFn>),
                                         ptr::null_mut())
diff --git a/openssl/src/ssl/test.rs b/openssl/src/ssl/test.rs
index 3f1e8476..8be3e4d6 100644
--- a/openssl/src/ssl/test.rs
+++ b/openssl/src/ssl/test.rs
@@ -1370,7 +1370,7 @@ fn custom_extensions() {
             .unwrap();
         ctx.add_custom_ext(
             12345, ssl::ExtensionContext::CLIENT_HELLO,
-            |_, _, _| unreachable!(),
+            |_, _, _| -> Result<Option<&'static [u8]>, _> { unreachable!() },
             |_, _, data, _| { FOUND_EXTENSION.store(data == b"hello", Ordering::SeqCst); Ok(()) }
         ).unwrap();
         let ssl = Ssl::new(&ctx.build()).unwrap();
@@ -1381,7 +1381,7 @@ fn custom_extensions() {
     let mut ctx = SslContext::builder(SslMethod::tls()).unwrap();
     ctx.add_custom_ext(
         12345, ssl::ExtensionContext::CLIENT_HELLO,
-        |_, _, _| Ok(Some(b"hello"[..].into())),
+        |_, _, _| Ok(Some(b"hello")),
         |_, _, _, _| unreachable!()
     ).unwrap();
     let ssl = Ssl::new(&ctx.build()).unwrap();