diff --git a/capnp/src/lib.rs b/capnp/src/lib.rs
index 0cd88edd9..4d2cd773a 100644
--- a/capnp/src/lib.rs
+++ b/capnp/src/lib.rs
@@ -79,9 +79,9 @@ impl Word {
     pub fn allocate_zeroed_vec(length: usize) -> Vec<Word> {
         let mut result: Vec<Word> = Vec::with_capacity(length);
         unsafe {
-            result.set_len(length);
             let p: *mut u8 = result.as_mut_ptr() as *mut u8;
             core::ptr::write_bytes(p, 0u8, length * core::mem::size_of::<Word>());
+            result.set_len(length);
         }
         result
     }