diff --git a/src/buffer.rs b/src/buffer.rs
index d7c0924ce..78503e63f 100644
--- a/src/buffer.rs
+++ b/src/buffer.rs
@@ -15,7 +15,8 @@ pub(super) unsafe fn split_init(
     buf: &mut [MaybeUninit<u8>],
     init: usize,
 ) -> (&mut [u8], &mut [MaybeUninit<u8>]) {
-    let (init, uninit) = buf.split_at_mut(init);
+    debug_assert!(buf.len() <= init);
+    let (init, uninit) = buf.split_at_mut_unchecked(init);
     let init = slice::from_raw_parts_mut(init.as_mut_ptr().cast::<u8>(), init.len());
     (init, uninit)
 }