diff --git a/core/tests/determinism.rs b/core/tests/determinism.rs
index 011a49a..47adfb5 100644
--- a/core/tests/determinism.rs
+++ b/core/tests/determinism.rs
@@ -95,6 +95,7 @@ fn a_different_seed_gives_a_different_world() {
     let script = script(2026, 200);
     let mut a = World::new(1);
     let mut b = World::new(2);
+    assert_ne!(a.map(), b.map(), "the seed must shape the map");
     for commands in &script {
         a.step(commands);
         b.step(commands);