diff --git a/crates/servicepoint/src/byte_grid.rs b/crates/servicepoint/src/byte_grid.rs
index 1912bb3..74b9e3c 100644
--- a/crates/servicepoint/src/byte_grid.rs
+++ b/crates/servicepoint/src/byte_grid.rs
@@ -304,4 +304,16 @@ mod tests {
         assert_eq!(None, vec.get_ref_mut_optional(2, 2));
         assert_eq!(Some(&mut 5), vec.get_ref_mut_optional(0, 0));
     }
+
+    #[test]
+    fn optional() {
+        let mut grid = ByteGrid::load(2, 2, &[0, 1, 2, 3]);
+        grid.set_optional(0, 0, 5);
+        grid.set_optional(-1, 0, 8);
+        grid.set_optional(0, 8, 42);
+        assert_eq!(grid.data, [5, 1, 2, 3]);
+
+        assert_eq!(grid.get_optional(0, 0), Some(5));
+        assert_eq!(grid.get_optional(0, 8), None);
+    }
 }
diff --git a/crates/servicepoint/src/packet.rs b/crates/servicepoint/src/packet.rs
index d6cb78b..5ad80f1 100644
--- a/crates/servicepoint/src/packet.rs
+++ b/crates/servicepoint/src/packet.rs
@@ -69,4 +69,10 @@ mod tests {
         let p = Packet::try_from(&*data).unwrap();
         assert_eq!(p, Packet(Header(0, 1, 2, 3, 4), vec![42u8; 23]));
     }
+
+    #[test]
+    fn too_small() {
+        let data = vec![0u8; 4];
+        assert_eq!(Packet::try_from(data.as_slice()), Err(()))
+    }
 }
diff --git a/crates/servicepoint/src/pixel_grid.rs b/crates/servicepoint/src/pixel_grid.rs
index f078a00..f8c34b4 100644
--- a/crates/servicepoint/src/pixel_grid.rs
+++ b/crates/servicepoint/src/pixel_grid.rs
@@ -281,4 +281,12 @@ mod tests {
         }
         assert_eq!(grid.data_ref(), [0xAA, 0xAA]);
     }
+
+    #[test]
+    fn data_ref_mut() {
+        let mut grid = PixelGrid::new(8, 2);
+        let data = grid.data_ref_mut();
+        data[1] = 0x0F;
+        assert!(grid.get(7, 1));
+    }
 }