diff --git a/chain/chain_manager.go b/chain/chain_manager.go
index 60de377aa7..dd965e85bd 100644
--- a/chain/chain_manager.go
+++ b/chain/chain_manager.go
@@ -319,10 +319,7 @@ func (self *ChainManager) InsertChain(chain *BlockChain) {
 	}
 }
 
-func (self *ChainManager) TestChain(chain *BlockChain) (i int, err error) {
-	var (
-		td *big.Int
-	)
+func (self *ChainManager) TestChain(chain *BlockChain) (td *big.Int, err error) {
 	for e := chain.Front(); e != nil; e = e.Next() {
 		var (
 			l      = e.Value.(*link)
@@ -355,9 +352,9 @@ func (self *ChainManager) TestChain(chain *BlockChain) (i int, err error) {
 	}
 
 	if td.Cmp(self.TD) <= 0 {
-		err = fmt.Errorf("incoming chain has a lower or equal TD (%v <= %v)", td, self.TD)
+		err = &TDError{td, self.TD}
 		return
 	}
 
-	return i, nil
+	return
 }
diff --git a/chain/error.go b/chain/error.go
index 204b8b8738..71bda8e7b9 100644
--- a/chain/error.go
+++ b/chain/error.go
@@ -114,3 +114,15 @@ func IsOutOfGasErr(err error) bool {
 
 	return ok
 }
+
+type TDError struct {
+	a, b *big.Int
+}
+
+func (self *TDError) Error() string {
+	return fmt.Sprintf("incoming chain has a lower or equal TD (%v <= %v)", self.a, self.b)
+}
+func IsTDError(e error) bool {
+	_, ok := err.(*TDError)
+	return ok
+}