diff --git a/crates/net/rpc/src/beacon/ptc.rs b/crates/net/rpc/src/beacon/ptc.rs
index be80bf330..1a00631e4 100644
--- a/crates/net/rpc/src/beacon/ptc.rs
+++ b/crates/net/rpc/src/beacon/ptc.rs
@@ -533,7 +533,7 @@ fn aggregate_pool(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::test_utils::{RecordingNetwork, gloas_beacon_block};
+    use crate::test_utils::{RecordingNetwork, gloas_beacon_block, slot_root};
     use axum::{body::Body, http::Request};
     use ethlambda_state_transition::beacon::{
         constants::DOMAIN_PTC_ATTESTER,
@@ -571,7 +571,7 @@ mod tests {
         // Distinct roots per slot, so a `dependent_root` read from the wrong
         // slot shows up.
         for slot in 0..inner.block_roots.len() {
-            inner.block_roots[slot] = H256::repeat_byte(slot as u8 + 1);
+            inner.block_roots[slot] = slot_root(slot as u64);
         }
         for index in 0..preset::PTC_WINDOW_LENGTH {
             inner.ptc_window[index] = vec![index as u64; preset::PTC_SIZE]
@@ -739,7 +739,7 @@ mod tests {
             let expected = if dependent_slot >= head_slot {
                 head_root
             } else {
-                H256::repeat_byte(dependent_slot as u8 + 1)
+                slot_root(dependent_slot)
             };
             assert_eq!(json["dependent_root"], format!("{expected:?}"));
         }
diff --git a/crates/net/rpc/src/beacon/validator.rs b/crates/net/rpc/src/beacon/validator.rs
index ee2f0bcde..043f3bd2a 100644
--- a/crates/net/rpc/src/beacon/validator.rs
+++ b/crates/net/rpc/src/beacon/validator.rs
@@ -866,7 +866,7 @@ fn payload_present_index(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::test_utils::{beacon_store_at, idle_engine};
+    use crate::test_utils::{beacon_store_at, idle_engine, slot_root};
     use axum::{
         body::Body,
         http::{Request, StatusCode},
@@ -893,7 +893,7 @@ mod tests {
         };
         fulu.proposer_lookahead = lookahead.try_into().unwrap();
         for slot in 0..fulu.block_roots.len() {
-            fulu.block_roots[slot] = H256::repeat_byte(slot as u8 + 1);
+            fulu.block_roots[slot] = slot_root(slot as u64);
         }
         state
     }
diff --git a/crates/net/rpc/src/lib.rs b/crates/net/rpc/src/lib.rs
index 4870c2781..36481df1c 100644
--- a/crates/net/rpc/src/lib.rs
+++ b/crates/net/rpc/src/lib.rs
@@ -752,6 +752,19 @@ pub(crate) mod test_utils {
         Some(client.expect("building a client opens no connection"))
     }
 
+    /// A block root of its own for every `slot`, never the zero hash: `slot + 1`
+    /// in the last eight bytes, big-endian.
+    ///
+    /// For filling a state's `block_roots` so a root read from the wrong slot
+    /// shows up. `H256::repeat_byte(slot as u8 + 1)` cannot do that: the window
+    /// is `SLOTS_PER_HISTORICAL_ROOT` long, more slots than a byte counts, so it
+    /// overflows at slot 255 (a panic in debug, repeating roots in release).
+    pub(crate) fn slot_root(slot: u64) -> H256 {
+        let mut root = [0; 32];
+        root[24..].copy_from_slice(&(slot + 1).to_be_bytes());
+        H256(root)
+    }
+
     /// A beacon store whose anchor, and so head, is `state`, under a block at
     /// the state's slot. Returns the store and that block's root.
     ///