diff --git a/specs/phase1/custody-game.md b/specs/phase1/custody-game.md
index 24d64b0ee..5ee38be8b 100644
--- a/specs/phase1/custody-game.md
+++ b/specs/phase1/custody-game.md
@@ -519,8 +519,10 @@ def process_custody_slashing(state: BeaconState, signed_custody_slashing: Signed
     shard_transition = custody_slashing.shard_transition
     assert hash_tree_root(shard_transition) == attestation.data.shard_transition_root
     # Verify that the provided data matches the shard-transition
-    assert custody_slashing.data.get_backing().get_left().merkle_root() \
+    assert (
+        custody_slashing.data.get_backing().get_left().merkle_root()
         == shard_transition.shard_data_roots[custody_slashing.data_index]
+    )
     assert len(custody_slashing.data) == shard_transition.shard_block_lengths[custody_slashing.data_index]
 
     # Verify existence and participation of claimed malefactor