diff --git a/crates/omnigraph-server/examples/bench_actor_isolation.rs b/crates/omnigraph-server/examples/bench_actor_isolation.rs
index 96e9cec..c4ffd8d 100644
--- a/crates/omnigraph-server/examples/bench_actor_isolation.rs
+++ b/crates/omnigraph-server/examples/bench_actor_isolation.rs
@@ -158,7 +158,10 @@ async fn drive_heavy_actor(
 ) -> (usize, usize, usize) {
     use tokio::sync::Semaphore;
 
-    let limiter = Arc::new(Semaphore::new(concurrency.max(1)));
+    // Asserted at startup in `main()`; check again here for defense in
+    // depth so a future caller can't pass 0 silently.
+    assert!(concurrency > 0, "drive_heavy_actor concurrency must be > 0");
+    let limiter = Arc::new(Semaphore::new(concurrency));
     let mut handles = Vec::with_capacity(batches);
     for b in 0..batches {
         let app = app.clone();
@@ -251,6 +254,13 @@ async fn main() {
         eprintln!("--light-actors, --light-ops-per-actor, --heavy-batches must all be > 0");
         std::process::exit(2);
     }
+    if args.heavy_concurrency == 0 {
+        eprintln!(
+            "--heavy-concurrency must be > 0 (zero would prevent the heavy actor from \
+             ever firing a batch; if you want to disable heavy traffic, set --heavy-batches=0)"
+        );
+        std::process::exit(2);
+    }
 
     let temp = tempfile::tempdir().expect("tempdir");
     let repo = temp.path().join("bench.omni");