diff --git a/listings/ch20-advanced-features/listing-20-06/src/main.rs b/listings/ch20-advanced-features/listing-20-06/src/main.rs
index 3af21f7614..80be88b3e0 100644
--- a/listings/ch20-advanced-features/listing-20-06/src/main.rs
+++ b/listings/ch20-advanced-features/listing-20-06/src/main.rs
@@ -5,7 +5,7 @@ fn split_at_mut(values: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) {
     let len = values.len();
     let ptr = values.as_mut_ptr();
 
-    assert!(mid <= len);
+    assert!(mid < len);
 
     unsafe {
         (