diff --git a/src/payload.rs b/src/payload.rs
index cc2695c..685347b 100644
--- a/src/payload.rs
+++ b/src/payload.rs
@@ -1382,8 +1382,23 @@ mod test {
 
     #[test]
     fn set_merge() {
+        assert!(
+            set([block([], 0..0)]).merge(
+                &set([block([], 0..0)])
+            ).iter().eq(set([block([], 0..0)]).iter())
+        );
+        assert!(
+            set([block([1, 3, 4], 0..3)]).merge(
+                &set([block([1, 3, 4], 0..3)])
+            ).iter().eq(set([block([1, 3, 4], 0..3)]).iter())
+        );
         assert!(
             set([block([1, 3, 4], 0..3)]).merge(
+                &set([block([], 0..0)])
+            ).iter().eq(set([block([1, 3, 4], 0..3)]).iter())
+        );
+        assert!(
+            set([block([], 0..0)]).merge(
                 &set([block([1, 3, 4], 0..3)])
             ).iter().eq(set([block([1, 3, 4], 0..3)]).iter())
         );