diff --git a/src/custom.rs b/src/custom.rs
index 35771f15..c4a25dc6 100644
--- a/src/custom.rs
+++ b/src/custom.rs
@@ -78,9 +78,9 @@ macro_rules! register_custom_getrandom {
     ($path:path) => {
         // We use an extern "C" function to get the guarantees of a stable ABI.
         #[no_mangle]
-        extern "C" fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
+        unsafe extern "C" fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
             let f: fn(&mut [u8]) -> Result<(), $crate::Error> = $path;
-            let slice = unsafe { ::core::slice::from_raw_parts_mut(dest, len) };
+            let slice = ::core::slice::from_raw_parts_mut(dest, len);
             match f(slice) {
                 Ok(()) => 0,
                 Err(e) => e.code().get(),