diff --git a/beacon_node/beacon_chain/src/test_utils.rs b/beacon_node/beacon_chain/src/test_utils.rs
index 5e61fb79a80..67a53b90497 100644
--- a/beacon_node/beacon_chain/src/test_utils.rs
+++ b/beacon_node/beacon_chain/src/test_utils.rs
@@ -229,6 +229,29 @@ impl<E: EthSpec> BeaconChainHarness<EphemeralHarnessType<E>> {
         target_aggregators_per_committee: u64,
         store_config: StoreConfig,
         chain_config: ChainConfig,
+    ) -> Self {
+        Self::new_with_mutator(
+            eth_spec_instance,
+            spec,
+            validator_keypairs,
+            target_aggregators_per_committee,
+            store_config,
+            chain_config,
+            |x| x,
+        )
+    }
+
+    /// Apply a function to beacon chain builder before building.
+    pub fn new_with_mutator(
+        eth_spec_instance: E,
+        spec: Option<ChainSpec>,
+        validator_keypairs: Vec<Keypair>,
+        target_aggregators_per_committee: u64,
+        store_config: StoreConfig,
+        chain_config: ChainConfig,
+        mutator: impl FnOnce(
+            BeaconChainBuilder<EphemeralHarnessType<E>>,
+        ) -> BeaconChainBuilder<EphemeralHarnessType<E>>,
     ) -> Self {
         let data_dir = tempdir().expect("should create temporary data_dir");
         let mut spec = spec.unwrap_or_else(test_spec::<E>);
@@ -240,7 +263,7 @@ impl<E: EthSpec> BeaconChainHarness<EphemeralHarnessType<E>> {
         let log = test_logger();
 
         let store = HotColdDB::open_ephemeral(store_config, spec.clone(), log.clone()).unwrap();
-        let chain = BeaconChainBuilder::new(eth_spec_instance)
+        let builder = BeaconChainBuilder::new(eth_spec_instance)
             .logger(log.clone())
             .custom_spec(spec.clone())
             .store(Arc::new(store))
@@ -260,9 +283,9 @@ impl<E: EthSpec> BeaconChainHarness<EphemeralHarnessType<E>> {
                 log.clone(),
                 1,
             )))
-            .monitor_validators(true, vec![], log)
-            .build()
-            .expect("should build");
+            .monitor_validators(true, vec![], log);
+
+        let chain = mutator(builder).build().expect("should build");
 
         Self {
             spec: chain.spec.clone(),
diff --git a/beacon_node/beacon_chain/tests/block_verification.rs b/beacon_node/beacon_chain/tests/block_verification.rs
index 446b19195ab..ccdb98a2310 100644
--- a/beacon_node/beacon_chain/tests/block_verification.rs
+++ b/beacon_node/beacon_chain/tests/block_verification.rs
@@ -4,7 +4,7 @@
 extern crate lazy_static;
 
 use beacon_chain::test_utils::{
-    AttestationStrategy, BeaconChainHarness, BlockStrategy, EphemeralHarnessType,
+    test_logger, AttestationStrategy, BeaconChainHarness, BlockStrategy, EphemeralHarnessType,
 };
 use beacon_chain::{BeaconSnapshot, BlockError, ChainConfig, ChainSegmentResult};
 use slasher::{Config as SlasherConfig, Slasher};
@@ -830,17 +830,25 @@ fn block_gossip_verification() {
 
 #[test]
 fn verify_block_for_gossip_slashing_detection() {
-    let mut harness = get_harness(VALIDATOR_COUNT);
-
     let slasher_dir = tempdir().unwrap();
     let slasher = Arc::new(
         Slasher::open(
             SlasherConfig::new(slasher_dir.path().into()).for_testing(),
-            harness.logger().clone(),
+            test_logger(),
         )
         .unwrap(),
     );
-    harness.chain.slasher = Some(slasher.clone());
+
+    let harness = BeaconChainHarness::new_with_mutator(
+        MainnetEthSpec,
+        None,
+        KEYPAIRS.to_vec(),
+        1 << 32,
+        StoreConfig::default(),
+        ChainConfig::default(),
+        |builder| builder.slasher(slasher.clone()),
+    );
+    harness.advance_slot();
 
     let state = harness.get_current_state();
     let (block1, _) = harness.make_block(state.clone(), Slot::new(1));