diff --git a/core/rawdb/accessors_offchain.go b/core/rawdb/accessors_offchain.go
index d3ee79d99..1789d917b 100644
--- a/core/rawdb/accessors_offchain.go
+++ b/core/rawdb/accessors_offchain.go
@@ -1,6 +1,7 @@
 package rawdb
 
 import (
+	"fmt"
 	"math/big"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -301,7 +302,7 @@ func ReadBlockCommitSig(db DatabaseReader, blockNum uint64) ([]byte, error) {
 		//       this is only needed for the compatibility in the migration moment.
 		data, err = db.Get(lastCommitsKey)
 		if err != nil {
-			return nil, errors.New("cannot read commit sig for block " + string(blockNum))
+			return nil, errors.New(fmt.Sprintf("cannot read commit sig for block: %d ", blockNum))
 		}
 	}
 	return data, nil