diff --git a/eth/filters/filter.go b/eth/filters/filter.go
index 071613ad7a..6b50cea670 100644
--- a/eth/filters/filter.go
+++ b/eth/filters/filter.go
@@ -30,6 +30,7 @@ import (
 	"github.com/ethereum/go-ethereum/rpc"
 )
 
+// Backend interface
 type Backend interface {
 	ChainDb() ethdb.Database
 	EventMux() *event.TypeMux
diff --git a/eth/filters/filter_system.go b/eth/filters/filter_system.go
index 70139c1a96..586ef2005b 100644
--- a/eth/filters/filter_system.go
+++ b/eth/filters/filter_system.go
@@ -53,7 +53,7 @@ const (
 	PendingTransactionsSubscription
 	// BlocksSubscription queries hashes for blocks that are imported
 	BlocksSubscription
-	// LastSubscription keeps track of the last index
+	// LastIndexSubscription keeps track of the last index
 	LastIndexSubscription
 )
 
@@ -71,6 +71,7 @@ const (
 )
 
 var (
+	// ErrInvalidSubscriptionID is the invalid id error
 	ErrInvalidSubscriptionID = errors.New("invalid id")
 )