diff --git a/src/lib.rs b/src/lib.rs
index aa98023077..43f009f0cd 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1503,7 +1503,7 @@ pub unsafe trait TryFromBytes {
     doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.FromZeros.html"),
     doc = concat!("[derive-analysis]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.FromZeros.html#analysis"),
 )]
-pub unsafe trait FromZeros {
+pub unsafe trait FromZeros: TryFromBytes {
     // The `Self: Sized` bound makes it so that `FromZeros` is still object
     // safe.
     #[doc(hidden)]
@@ -3774,8 +3774,10 @@ safety_comment! {
     ///
     ///   `UnsafeCell<T>`` has the same in-memory representation as its inner
     ///   type `T`.
-    unsafe_impl!(T: ?Sized + FromZeros => FromZeros for UnsafeCell<T>);
-    unsafe_impl!(T: ?Sized + FromBytes => FromBytes for UnsafeCell<T>);
+    ///
+    /// TODO(#5): Implement `FromZeros` and `FromBytes` when `T: ?Sized`.
+    unsafe_impl!(T: FromZeros => FromZeros for UnsafeCell<T>);
+    unsafe_impl!(T: FromBytes => FromBytes for UnsafeCell<T>);
     unsafe_impl!(T: ?Sized + IntoBytes => IntoBytes for UnsafeCell<T>);
     unsafe_impl!(T: ?Sized + Unaligned => Unaligned for UnsafeCell<T>);
     assert_unaligned!(UnsafeCell<()>, UnsafeCell<u8>);
@@ -7001,7 +7003,6 @@ mod tests {
     #[test]
     fn test_object_safety() {
         fn _takes_no_cell(_: &dyn NoCell) {}
-        fn _takes_from_zeros(_: &dyn FromZeros) {}
         fn _takes_unaligned(_: &dyn Unaligned) {}
     }
 
diff --git a/zerocopy-derive/src/lib.rs b/zerocopy-derive/src/lib.rs
index 6bce135351..3b2c0cf1e0 100644
--- a/zerocopy-derive/src/lib.rs
+++ b/zerocopy-derive/src/lib.rs
@@ -395,10 +395,12 @@ fn derive_try_from_bytes_struct(ast: &DeriveInput, strct: &DataStruct) -> proc_m
 }
 
 // A union is `TryFromBytes` if:
-// - any of its fields are `TryFromBytes`
+// - all of its fields are `TryFromBytes` and `NoCell`
 
 fn derive_try_from_bytes_union(ast: &DeriveInput, unn: &DataUnion) -> proc_macro2::TokenStream {
-    let self_type_trait_bounds = SelfBounds::All(&[Trait::NoCell]);
+    // TODO(#5): Remove the `NoCell` bound.
+    let field_type_trait_bounds =
+        FieldBounds::All(&[TraitBound::Slf, TraitBound::Other(Trait::NoCell)]);
     let extras = Some({
         let fields = unn.fields();
         let field_names = fields.iter().map(|(name, _ty)| name);
@@ -434,8 +436,8 @@ fn derive_try_from_bytes_union(ast: &DeriveInput, unn: &DataUnion) -> proc_macro
         ast,
         unn,
         Trait::TryFromBytes,
-        FieldBounds::ALL_SELF,
-        self_type_trait_bounds,
+        field_type_trait_bounds,
+        SelfBounds::None,
         None,
         extras,
     )
@@ -606,11 +608,15 @@ fn derive_from_zeros_enum(ast: &DeriveInput, enm: &DataEnum) -> proc_macro2::Tok
     impl_block(ast, enm, Trait::FromZeros, FieldBounds::ALL_SELF, SelfBounds::None, None, None)
 }
 
-// Like structs, unions are `FromZeros` if
-// - all fields are `FromZeros`
+// Unions are `FromZeros` if
+// - all fields are `FromZeros` and `NoCell`
 
 fn derive_from_zeros_union(ast: &DeriveInput, unn: &DataUnion) -> proc_macro2::TokenStream {
-    impl_block(ast, unn, Trait::FromZeros, FieldBounds::ALL_SELF, SelfBounds::None, None, None)
+    // TODO(#5): Remove the `NoCell` bound. It's only necessary for
+    // compatibility with `derive(TryFromBytes)` on unions; not for soundness.
+    let field_type_trait_bounds =
+        FieldBounds::All(&[TraitBound::Slf, TraitBound::Other(Trait::NoCell)]);
+    impl_block(ast, unn, Trait::FromZeros, field_type_trait_bounds, SelfBounds::None, None, None)
 }
 
 // A struct is `FromBytes` if:
@@ -692,11 +698,15 @@ const ENUM_FROM_BYTES_CFG: Config<EnumRepr> = {
     }
 };
 
-// Like structs, unions are `FromBytes` if
-// - all fields are `FromBytes`
+// Unions are `FromBytes` if
+// - all fields are `FromBytes` and `NoCell`
 
 fn derive_from_bytes_union(ast: &DeriveInput, unn: &DataUnion) -> proc_macro2::TokenStream {
-    impl_block(ast, unn, Trait::FromBytes, FieldBounds::ALL_SELF, SelfBounds::None, None, None)
+    // TODO(#5): Remove the `NoCell` bound. It's only necessary for
+    // compatibility with `derive(TryFromBytes)` on unions; not for soundness.
+    let field_type_trait_bounds =
+        FieldBounds::All(&[TraitBound::Slf, TraitBound::Other(Trait::NoCell)]);
+    impl_block(ast, unn, Trait::FromBytes, field_type_trait_bounds, SelfBounds::None, None, None)
 }
 
 fn derive_into_bytes_struct(ast: &DeriveInput, strct: &DataStruct) -> proc_macro2::TokenStream {