diff --git a/glib-macros/src/boxed_derive.rs b/glib-macros/src/boxed_derive.rs
index ab51b68da0d9..c6e254b43582 100644
--- a/glib-macros/src/boxed_derive.rs
+++ b/glib-macros/src/boxed_derive.rs
@@ -192,7 +192,9 @@ pub fn impl_boxed(input: &syn::DeriveInput) -> syn::Result<TokenStream> {
 
         #impl_from_value
 
-        unsafe impl #crate_ident::translate::TransparentPtrType for #name {}
+        unsafe impl #crate_ident::translate::TransparentType for #name {
+            type GlibType = #name;
+        }
 
         impl #crate_ident::translate::GlibPtrDefault for #name {
             type GlibType = *mut #name;
diff --git a/glib-macros/tests/test.rs b/glib-macros/tests/test.rs
index 95471891ded5..c5e40237a02a 100644
--- a/glib-macros/tests/test.rs
+++ b/glib-macros/tests/test.rs
@@ -1,7 +1,7 @@
 // Take a look at the license at the top of the repository in the LICENSE file.
 
 use glib::{
-    collections::PtrSlice,
+    collections::Slice,
     prelude::*,
     translate::{FromGlib, IntoGlib},
 };
@@ -185,19 +185,16 @@ fn derive_boxed_nullable() {
 }
 
 #[test]
-fn boxed_transparent_ptr() {
+fn boxed_transparent() {
     #[derive(Clone, Debug, PartialEq, Eq, glib::Boxed)]
     #[boxed_type(name = "MyBoxed")]
     struct MyBoxed(String);
 
     let vec = vec![MyBoxed(String::from("abc")), MyBoxed(String::from("dfg"))];
 
-    // PtrSlice requires TransparentPtrType trait
-    let ptr_slice = PtrSlice::from(vec);
-    assert_eq!(
-        ptr_slice.last(),
-        Some(MyBoxed(String::from("dfg"))).as_ref()
-    );
+    // Slice requires TransparentType trait
+    let slice = Slice::from(vec);
+    assert_eq!(slice.last(), Some(MyBoxed(String::from("dfg"))).as_ref());
 }
 
 #[test]
diff --git a/glib/src/collections/ptr_slice.rs b/glib/src/collections/ptr_slice.rs
index 5788e0e4b603..4c19b0655ad5 100644
--- a/glib/src/collections/ptr_slice.rs
+++ b/glib/src/collections/ptr_slice.rs
@@ -601,6 +601,11 @@ impl<T: TransparentPtrType> PtrSlice<T> {
     /// Creates a new empty slice.
     #[inline]
     pub fn new() -> Self {
+        debug_assert_eq!(
+            mem::size_of::<T>(),
+            mem::size_of::<<T as GlibPtrDefault>::GlibType>()
+        );
+
         PtrSlice {
             ptr: ptr::NonNull::dangling(),
             len: 0,
diff --git a/glib/src/collections/slice.rs b/glib/src/collections/slice.rs
index 74c30a4f0938..eb674c342480 100644
--- a/glib/src/collections/slice.rs
+++ b/glib/src/collections/slice.rs
@@ -403,6 +403,7 @@ impl<T: TransparentType> Slice<T> {
     /// Borrows a C array.
     #[inline]
     pub unsafe fn from_glib_borrow_num<'a>(ptr: *const T::GlibType, len: usize) -> &'a [T] {
+        debug_assert_eq!(mem::size_of::<T>(), mem::size_of::<T::GlibType>());
         debug_assert!(!ptr.is_null() || len == 0);
 
         if len == 0 {
@@ -416,6 +417,7 @@ impl<T: TransparentType> Slice<T> {
     /// Borrows a mutable C array.
     #[inline]
     pub unsafe fn from_glib_borrow_num_mut<'a>(ptr: *mut T::GlibType, len: usize) -> &'a mut [T] {
+        debug_assert_eq!(mem::size_of::<T>(), mem::size_of::<T::GlibType>());
         debug_assert!(!ptr.is_null() || len == 0);
 
         if len == 0 {
@@ -432,6 +434,7 @@ impl<T: TransparentType> Slice<T> {
         ptr: *const *const T::GlibType,
         len: usize,
     ) -> &'a [&'a T] {
+        debug_assert_eq!(mem::size_of::<T>(), mem::size_of::<T::GlibType>());
         debug_assert!(!ptr.is_null() || len == 0);
 
         if len == 0 {
@@ -448,6 +451,7 @@ impl<T: TransparentType> Slice<T> {
         ptr: *mut *mut T::GlibType,
         len: usize,
     ) -> &'a mut [&'a mut T] {
+        debug_assert_eq!(mem::size_of::<T>(), mem::size_of::<T::GlibType>());
         debug_assert!(!ptr.is_null() || len == 0);
 
         if len == 0 {
@@ -522,6 +526,8 @@ impl<T: TransparentType> Slice<T> {
     /// Creates a new empty slice.
     #[inline]
     pub fn new() -> Self {
+        debug_assert_eq!(mem::size_of::<T>(), mem::size_of::<T::GlibType>());
+
         Slice {
             ptr: ptr::NonNull::dangling(),
             len: 0,