diff --git a/src/aes_ctr.rs b/src/aes_ctr.rs
index 44bcdb48..0f34335c 100644
--- a/src/aes_ctr.rs
+++ b/src/aes_ctr.rs
@@ -140,7 +140,7 @@ pub trait AesCipher {
 /// XORs a slice in place with another slice.
 #[inline]
 fn xor(dest: &mut [u8], src: &[u8]) {
-    debug_assert_eq!(dest.len(), src.len());
+    assert_eq!(dest.len(), src.len());
 
     for (lhs, rhs) in dest.iter_mut().zip(src.iter()) {
         *lhs ^= *rhs;