diff --git a/src/custom.rs b/src/custom.rs
index 923b1cf8..fc216e08 100644
--- a/src/custom.rs
+++ b/src/custom.rs
@@ -1,14 +1,10 @@
 //! An implementation which calls out to an externally defined function.
 use crate::Error;
-use core::{mem::MaybeUninit, num::NonZeroU32};
+use core::mem::MaybeUninit;
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     extern "Rust" {
-        fn __getrandom_custom(dest: *mut u8, len: usize) -> u32;
-    }
-    let ret = unsafe { __getrandom_custom(dest.as_mut_ptr().cast(), dest.len()) };
-    match NonZeroU32::new(ret) {
-        None => Ok(()),
-        Some(code) => Err(Error::from(code)),
+        fn __getrandom_custom(dest: *mut u8, len: usize) -> Result<(), Error>;
     }
+    unsafe { __getrandom_custom(dest.as_mut_ptr().cast(), dest.len()) }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 62c2dab3..aeba316a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -106,7 +106,7 @@
 //!
 //! ```
 //! #[no_mangle]
-//! unsafe fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
+//! unsafe fn __getrandom_custom(dest: *mut u8, len: usize) -> Result<(), getrandom::Error> {
 //!     todo!()
 //! }
 //! ```
@@ -127,8 +127,8 @@
 //! use the following custom backend which always returns "unsupported" error:
 //! ```
 //! #[no_mangle]
-//! unsafe fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
-//!     getrandom::Error::UNSUPPORTED.code().get()
+//! unsafe fn __getrandom_custom(dest: *mut u8, len: usize) -> Result<(), getrandom::Error> {
+//!     Err(getrandom::Error::UNSUPPORTED)
 //! }
 //! ```
 //!