diff --git a/objc2/src/declare.rs b/objc2/src/declare.rs
index 691d5b197..9bdb02f3a 100644
--- a/objc2/src/declare.rs
+++ b/objc2/src/declare.rs
@@ -45,7 +45,7 @@ use crate::{ffi, Encode, EncodeArguments, Encoding, Message};
 /// Types that can be used as the implementation of an Objective-C method.
 pub trait MethodImplementation {
     /// The callee type of the method.
-    type Callee: Message;
+    type Callee: Message + ?Sized;
     /// The return type of the method.
     type Ret: Encode;
     /// The argument types of the method.
@@ -59,7 +59,7 @@ macro_rules! method_decl_impl {
     (-$s:ident, $r:ident, $f:ty, $($t:ident),*) => (
         impl<$s, $r, $($t),*> MethodImplementation for $f
         where
-            $s: Message,
+            $s: Message + ?Sized,
             $r: Encode,
             $($t: Encode,)*
         {
diff --git a/objc2/src/rc/autorelease.rs b/objc2/src/rc/autorelease.rs
index 8e5b99493..f22e3bc5e 100644
--- a/objc2/src/rc/autorelease.rs
+++ b/objc2/src/rc/autorelease.rs
@@ -202,7 +202,7 @@ auto_trait! {
 }
 
 #[cfg(not(feature = "unstable_autoreleasesafe"))]
-unsafe impl<T> AutoreleaseSafe for T {}
+unsafe impl<T: ?Sized> AutoreleaseSafe for T {}
 
 #[cfg(feature = "unstable_autoreleasesafe")]
 impl !AutoreleaseSafe for AutoreleasePool {}
diff --git a/objc2/src/runtime.rs b/objc2/src/runtime.rs
index cf339069d..3a3e388d9 100644
--- a/objc2/src/runtime.rs
+++ b/objc2/src/runtime.rs
@@ -580,17 +580,17 @@ impl Object {
 
 /// ```
 /// use objc2::runtime::Object;
-/// fn needs_nothing<T>() {}
+/// fn needs_nothing<T: ?Sized>() {}
 /// needs_nothing::<Object>();
 /// ```
 /// ```compile_fail
 /// use objc2::runtime::Object;
-/// fn needs_sync<T: Sync>() {}
+/// fn needs_sync<T: ?Sized + Sync>() {}
 /// needs_sync::<Object>();
 /// ```
 /// ```compile_fail
 /// use objc2::runtime::Object;
-/// fn needs_send<T: Send>() {}
+/// fn needs_send<T: ?Sized + Send>() {}
 /// needs_send::<Object>();
 /// ```
 #[cfg(doctest)]
@@ -734,7 +734,7 @@ mod tests {
 
     #[test]
     fn test_send_sync() {
-        fn assert_send_sync<T: Send + Sync>() {}
+        fn assert_send_sync<T: Send + Sync + ?Sized>() {}
         assert_send_sync::<Bool>();
         assert_send_sync::<Class>();
         assert_send_sync::<Ivar>();