diff --git a/consensus/system_contract/consensus.go b/consensus/system_contract/consensus.go
index 756e9250fdd2..40a8ccf9bdba 100644
--- a/consensus/system_contract/consensus.go
+++ b/consensus/system_contract/consensus.go
@@ -284,7 +284,15 @@ func (s *SystemContract) Seal(chain consensus.ChainHeaderReader, block *types.Bl
 	s.lock.RUnlock()
 
 	// Bail out if we're unauthorized to sign a block
-	// todo
+	expectedSigner, err := s.findSignerSlice(number)
+	if err != nil {
+		// e.g., "no signer for block"
+		return fmt.Errorf("cannot seal block %d: %w", number, err)
+	}
+
+	if signer != expectedSigner {
+		return errors.New("local node is not authorized to sign this block")
+	}
 
 	// Sweet, the protocol permits us to sign the block, wait for our time
 	delay := time.Unix(int64(header.Time), 0).Sub(time.Now()) // nolint: gosimple
diff --git a/consensus/system_contract/system_contract.go b/consensus/system_contract/system_contract.go
index 18064f8ff4b2..3b6bad3a9d07 100644
--- a/consensus/system_contract/system_contract.go
+++ b/consensus/system_contract/system_contract.go
@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"github.com/scroll-tech/go-ethereum"
 	"github.com/scroll-tech/go-ethereum/accounts/abi"
-	"github.com/scroll-tech/go-ethereum/ethclient"
 	"math/big"
 	"strings"
 	"sync"
@@ -96,7 +95,7 @@ func (s *SystemContract) fetchAllSigners(blockNum *big.Int) ([]SignerAddressL1,
 	}
 
 	// Perform the call via the ethclient interface (need to cast)
-	output, err := s.client.(*ethclient.Client).CallContract(s.ctx, msg, blockNum)
+	output, err := s.client.CallContract(s.ctx, msg, blockNum)
 	if err != nil {
 		return nil, fmt.Errorf("CallContract error: %w", err)
 	}
diff --git a/rollup/sync_service/types.go b/rollup/sync_service/types.go
index 08b4dda2ecf0..f3a0149568b6 100644
--- a/rollup/sync_service/types.go
+++ b/rollup/sync_service/types.go
@@ -20,4 +20,5 @@ type EthClient interface {
 	TransactionByHash(ctx context.Context, txHash common.Hash) (tx *types.Transaction, isPending bool, err error)
 	BlockByHash(ctx context.Context, hash common.Hash) (*types.Block, error)
 	StorageAt(ctx context.Context, account common.Address, key common.Hash, blockNumber *big.Int) ([]byte, error)
+	CallContract(ctx context.Context, msg ethereum.CallMsg, blockNumber *big.Int) ([]byte, error)
 }