diff --git a/src/proto_alpha/lib_protocol/test/integration/consensus/test_aggregate.ml b/src/proto_alpha/lib_protocol/test/integration/consensus/test_aggregate.ml
index f3663cebab3ebf0f5ba0547a14d10f361245424d..cd2fbffd88a788452616394a416424ba7c7f3b75 100644
--- a/src/proto_alpha/lib_protocol/test/integration/consensus/test_aggregate.ml
+++ b/src/proto_alpha/lib_protocol/test/integration/consensus/test_aggregate.ml
@@ -60,6 +60,10 @@ let conflicting_consensus_operation = function
   | Validate_errors.Consensus.Conflicting_consensus_operation _ -> true
   | _ -> false
 
+let unaggregated_eligible_attestation = function
+  | Validate_errors.Consensus.Unaggregated_eligible_attestation _ -> true
+  | _ -> false
+
 let find_aggregate_result receipt =
   let result_opt =
     List.find_map
@@ -327,6 +331,25 @@ let test_multiple_aggregations_per_block_forbidden () =
   let*! res = Block.bake ~operations:aggregates block in
   Assert.proto_error ~loc:__LOC__ res conflicting_consensus_operation
 
+let eligible_attestation_must_be_aggregated () =
+  let open Lwt_result_syntax in
+  let* _genesis, block =
+    init_genesis_with_some_bls_accounts ~aggregate_attestation:true ()
+  in
+  let* attesters = Context.get_attesters (B block) in
+  let attester = find_attester_with_bls_key attesters in
+  match attester with
+  | Some (attester, _) ->
+      let* operation = Op.attestation ~delegate:attester.consensus_key block in
+      (* Operation is valid in the Mempool *)
+      let* inc = Incremental.begin_construction ~mempool_mode:true block in
+      let* inc = Incremental.add_operation inc operation in
+      let* _ = Incremental.finalize_block inc in
+      (* Operation is invalid in a block *)
+      let*! res = Block.bake ~operation block in
+      Assert.proto_error ~loc:__LOC__ res unaggregated_eligible_attestation
+  | _ -> assert false
+
 let tests =
   [
     Tztest.tztest
@@ -357,6 +380,10 @@ let tests =
       "test_multiple_aggregations_per_block_forbidden"
       `Quick
       test_multiple_aggregations_per_block_forbidden;
+    Tztest.tztest
+      "eligible_attestation_must_be_aggregated"
+      `Quick
+      eligible_attestation_must_be_aggregated;
   ]
 
 let () =
diff --git a/src/proto_alpha/lib_protocol/validate.ml b/src/proto_alpha/lib_protocol/validate.ml
index 336e28593a6b77207d3dea4b9acd20cde577c3e4..8259229e5507a45acea29fdb87032f5e6fc9778d 100644
--- a/src/proto_alpha/lib_protocol/validate.ml
+++ b/src/proto_alpha/lib_protocol/validate.ml
@@ -787,7 +787,7 @@ module Consensus = struct
     in
     let* consensus_key, voting_power =
       match vi.mode with
-      | Application _ | Partial_validation _ | Construction _ ->
+      | Application _ | Partial_validation _ | Construction _ -> (
           let* () =
             check_block_attestation vi consensus_info consensus_content
           in
@@ -797,7 +797,14 @@ module Consensus = struct
               Attestation
               consensus_content.slot
           in
-          return (consensus_key, voting_power)
+          match (consensus_key.consensus_pk, dal_content) with
+          | Bls _, None when Constants.aggregate_attestation vi.ctxt ->
+              (* An attestation signed by a tz4 without DAL content must be
+                 included in an Attestations_aggregate, even when it is the only
+                 one in the quorum. *)
+              let hash = Operation.hash operation in
+              tzfail (Unaggregated_eligible_attestation hash)
+          | _ -> return (consensus_key, voting_power))
       | Mempool ->
           check_mempool_consensus
             vi
diff --git a/src/proto_alpha/lib_protocol/validate_errors.ml b/src/proto_alpha/lib_protocol/validate_errors.ml
index 0e3a0aea0a7329a8b3c832941ac7e0c61d6209e3..7e77d5e21ae102c44b6e89ebb1f27b09928a5d7c 100644
--- a/src/proto_alpha/lib_protocol/validate_errors.ml
+++ b/src/proto_alpha/lib_protocol/validate_errors.ml
@@ -134,6 +134,7 @@ module Consensus = struct
     | Aggregate_not_implemented
     | Non_bls_key_in_aggregate
     | Public_key_aggregation_failure
+    | Unaggregated_eligible_attestation of Operation_hash.t
 
   let () =
     register_error_kind
@@ -413,7 +414,22 @@ module Consensus = struct
       ~pp:(fun ppf () -> Format.fprintf ppf "Public key aggregation failed")
       Data_encoding.empty
       (function Public_key_aggregation_failure -> Some () | _ -> None)
-      (fun () -> Public_key_aggregation_failure)
+      (fun () -> Public_key_aggregation_failure) ;
+    register_error_kind
+      `Permanent
+      ~id:"validate.unaggregated_eligible_attestation"
+      ~title:"Unaggregated eligible attestation"
+      ~description:"An eligible attestation was found unaggregated"
+      ~pp:(fun ppf hash ->
+        Format.fprintf
+          ppf
+          "Attestation %a should have been aggregated."
+          Operation_hash.pp
+          hash)
+      Data_encoding.(obj1 (req "hash" Operation_hash.encoding))
+      (function
+        | Unaggregated_eligible_attestation hash -> Some hash | _ -> None)
+      (fun hash -> Unaggregated_eligible_attestation hash)
 end
 
 module Voting = struct
diff --git a/src/proto_alpha/lib_protocol/validate_errors.mli b/src/proto_alpha/lib_protocol/validate_errors.mli
index 2345124329c1d91eccea8e8f3199b1e4bc7e20be..1e48e261632865a7c3415816c33d0c208a4b6ac9 100644
--- a/src/proto_alpha/lib_protocol/validate_errors.mli
+++ b/src/proto_alpha/lib_protocol/validate_errors.mli
@@ -87,6 +87,7 @@ module Consensus : sig
     | Aggregate_not_implemented
     | Non_bls_key_in_aggregate
     | Public_key_aggregation_failure
+    | Unaggregated_eligible_attestation of Operation_hash.t
 end
 
 (** Errors that may arise while validating a voting operation. *)