diff --git a/specs/core/0_fork-choice.md b/specs/core/0_fork-choice.md
index 34948fbce..68e681fc7 100644
--- a/specs/core/0_fork-choice.md
+++ b/specs/core/0_fork-choice.md
@@ -233,9 +233,11 @@ def on_block(store: Store, block: BeaconBlock) -> None:
 def on_attestation(store: Store, attestation: Attestation) -> None:
     target = attestation.data.target
 
-    # Attestations must be from the current or previous epoch
+    # Attestations must be from the current or previous epoch 
     current_epoch = compute_epoch_at_slot(get_current_slot(store))
-    assert target.epoch in [current_epoch, current_epoch - 1 if current_epoch > GENESIS_EPOCH else GENESIS_EPOCH]
+    # Use GENESIS_EPOCH for previous when genesis to avoid underflow
+    previous_epoch = current_epoch - 1 if current_epoch > GENESIS_EPOCH else GENESIS_EPOCH
+    assert target.epoch in [current_epoch, previous_epoch]
     # Cannot calculate the current shuffling if have not seen the target
     assert target.root in store.blocks