diff --git a/specs/altair/p2p-interface.md b/specs/altair/p2p-interface.md
index e5e391ff5..6820fd10a 100644
--- a/specs/altair/p2p-interface.md
+++ b/specs/altair/p2p-interface.md
@@ -139,6 +139,8 @@ def get_sync_subcommittee_pubkeys(state: BeaconState, subcommittee_index: uint64
 
 - _[IGNORE]_ The contribution's slot is for the current slot (with a `MAXIMUM_GOSSIP_CLOCK_DISPARITY` allowance), i.e. `contribution.slot == current_slot`.
 - _[REJECT]_ The subcommittee index is in the allowed range, i.e. `contribution.subcommittee_index < SYNC_COMMITTEE_SUBNET_COUNT`.
+- _[REJECT]_ The contribution has participants --
+  that is, `any(contribution.aggregation_bits)`.
 - _[REJECT]_ `contribution_and_proof.selection_proof` selects the validator as an aggregator for the slot -- i.e. `is_sync_committee_aggregator(contribution_and_proof.selection_proof)` returns `True`.
 - _[REJECT]_ The aggregator's validator index is in the declared subcommittee of the current sync committee --
   i.e. `state.validators[contribution_and_proof.aggregator_index].pubkey in get_sync_subcommittee_pubkeys(state, contribution.subcommittee_index)`.
diff --git a/specs/altair/validator.md b/specs/altair/validator.md
index 8742900bc..badef2de5 100644
--- a/specs/altair/validator.md
+++ b/specs/altair/validator.md
@@ -354,7 +354,7 @@ def is_sync_committee_aggregator(signature: BLSSignature) -> bool:
 
 If a validator is selected to aggregate the `SyncCommitteeMessage`s produced on a subnet during a given `slot`, they construct an aggregated `SyncCommitteeContribution`.
 
-Given all of the (valid) collected `sync_committee_messages: Set[SyncCommitteeMessage]` from the `sync_committee_{subnet_id}` gossip during the selected `slot` with an equivalent `beacon_block_root` to that of the aggregator, the aggregator creates a `contribution: SyncCommitteeContribution` with the following fields:
+Collect all of the (valid) `sync_committee_messages: Set[SyncCommitteeMessage]` from the `sync_committee_{subnet_id}` gossip during the selected `slot` with an equivalent `beacon_block_root` to that of the aggregator. If `len(sync_committee_messages) > 0`, the aggregator creates a `contribution: SyncCommitteeContribution` with the following fields:
 
 ###### Slot