diff --git a/beacon_chain/attestation_aggregation.nim b/beacon_chain/attestation_aggregation.nim
index cceb30992..d3c964af2 100644
--- a/beacon_chain/attestation_aggregation.nim
+++ b/beacon_chain/attestation_aggregation.nim
@@ -76,7 +76,7 @@ proc aggregate_attestations*(
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/p2p-interface.md#attestation-subnets
 proc isValidAttestation*(
-    pool: AttestationPool, attestation: Attestation, current_slot: Slot,
+    pool: var AttestationPool, attestation: Attestation, current_slot: Slot,
     topicCommitteeIndex: uint64): bool =
   logScope:
     topics = "att_aggr valid_att"
@@ -135,6 +135,7 @@ proc isValidAttestation*(
   # therefore propagate faster, thus reordering their arrival in some nodes
   if pool.blockPool.get(attestation.data.beacon_block_root).isNone():
     debug "block doesn't exist in block pool"
+    pool.blockPool.addMissing(attestation.data.beacon_block_root)
     return false
 
   # The signature of attestation is valid.
diff --git a/beacon_chain/block_pool.nim b/beacon_chain/block_pool.nim
index 987e01857..cfe896d9c 100644
--- a/beacon_chain/block_pool.nim
+++ b/beacon_chain/block_pool.nim
@@ -127,6 +127,9 @@ proc latestJustifiedBlock*(pool: BlockPool): BlockSlot =
   ## as the latest finalized block
   latestJustifiedBlock(pool.dag)
 
+proc addMissing*(pool: var BlockPool, broot: Eth2Digest) {.inline.} =
+  pool.quarantine.addMissing(broot)
+
 proc isInitialized*(T: type BlockPool, db: BeaconChainDB): bool =
   isInitialized(CandidateChains, db)
 
diff --git a/beacon_chain/block_pools/quarantine.nim b/beacon_chain/block_pools/quarantine.nim
index 08851a973..3c4265883 100644
--- a/beacon_chain/block_pools/quarantine.nim
+++ b/beacon_chain/block_pools/quarantine.nim
@@ -39,6 +39,9 @@ func checkMissing*(quarantine: var Quarantine): seq[FetchRecord] =
     if countOnes(v.tries.uint64) == 1:
       result.add(FetchRecord(root: k))
 
+func addMissing*(quarantine: var Quarantine, broot: Eth2Digest) {.inline.} =
+  discard quarantine.missing.hasKeyOrPut(broot, MissingBlock())
+
 func add*(quarantine: var Quarantine, dag: CandidateChains,
           sblck: SignedBeaconBlock,
           broot: Option[Eth2Digest] = none[Eth2Digest]()) =
@@ -51,5 +54,4 @@ func add*(quarantine: var Quarantine, dag: CandidateChains,
   quarantine.orphans[blockRoot] = sblck
 
   let parentRoot = sblck.message.parent_root
-  if parentRoot notin quarantine.missing:
-    quarantine.missing[parentRoot] = MissingBlock()
+  quarantine.addMissing(parentRoot)