diff --git a/crates/cross/src/dim.rs b/crates/cross/src/dim.rs
index bb4063d..a354a78 100644
--- a/crates/cross/src/dim.rs
+++ b/crates/cross/src/dim.rs
@@ -70,7 +70,7 @@ mod test {
             3, 4, 5,
 
             0, 10,20,
-            10,40,50,
+            30,40,50,
         ];
         let s = DimensionedSlice::new(Vec3u::new(3, 2, 2), &data);
         assert_eq!(s[Vec3u::new(0, 0, 0)], 0);
@@ -79,5 +79,27 @@ mod test {
         assert_eq!(s[Vec3u::new(1, 1, 1)], 40);
         assert_eq!(s[Vec3u::new(2, 1, 1)], 50);
     }
+
+    #[test]
+    fn dimensioned_slice_index_mut() {
+        let mut data = [
+            0, 1, 2,
+            3, 4, 5,
+
+            0, 10,20,
+            30,40,50,
+        ];
+        let mut s = DimensionedSlice::new(Vec3u::new(3, 2, 2), &mut data);
+        s[Vec3u::new(0, 0, 0)] = 100;
+        s[Vec3u::new(0, 1, 1)] = 300;
+
+        assert_eq!(data, [
+            100,1, 2,
+            3,  4, 5,
+
+            0, 10, 20,
+            300,40,50,
+        ]);
+    }
 }