diff --git a/core/chain_manager.go b/core/chain_manager.go
index 29830188e3..59f3c20878 100644
--- a/core/chain_manager.go
+++ b/core/chain_manager.go
@@ -571,8 +571,15 @@ func (self *ChainManager) InsertChain(chain types.Blocks) (int, error) {
 			// Compare the TD of the last known block in the canonical chain to make sure it's greater.
 			// At this point it's possible that a different chain (fork) becomes the new canonical chain.
 			if block.Td.Cmp(self.td) > 0 {
+				//key := append(blockNumPre, block.Number().Bytes()...)
+				//self.blockDb.Put(key, block.Hash().Bytes())
 				// Check for chain forks. If H(block.num - 1) != block.parent, we're on a fork and need to do some merging
-				if previous := self.getBlockByNumber(block.NumberU64() - 1); previous.Hash() != block.ParentHash() {
+				previous := self.getBlockByNumber(block.NumberU64() - 1)
+				if previous == nil {
+					glog.V(logger.Error).Infof("INVALID block #%v (%x)\n", block.Header().Number, block.Header().Hash().Bytes())
+					return i, BlockNumberErr
+				}
+				if previous.Hash() != block.ParentHash() {
 					chash := cblock.Hash()
 					hash := block.Hash()
 
diff --git a/core/chain_manager_test.go b/core/chain_manager_test.go
index 50915459b9..486acf4f80 100644
--- a/core/chain_manager_test.go
+++ b/core/chain_manager_test.go
@@ -241,6 +241,24 @@ func TestBrokenChain(t *testing.T) {
 	}
 }
 
+func TestChainForkWithWrongBlockNum(t *testing.T) {
+	db, _ := ethdb.NewMemDatabase()
+	genesis := GenesisBlock(db)
+	bc := chm(genesis, db)
+
+	chain1 := makeChainWithDiff(genesis, []int{1, 2, 3}, 10)
+	chain2 := makeChainWithDiff(genesis, []int{4, 5}, 11)
+	chain2[0].Header().Number = big.NewInt(int64(6))
+	chain2[1].Header().Number = big.NewInt(int64(7))
+
+	_, _ = bc.InsertChain(chain1)
+	_, err := bc.InsertChain(chain2)
+
+	if err != BlockNumberErr {
+		t.Errorf("should have failed with BlockNumberErr")
+	}
+}
+
 func TestChainInsertions(t *testing.T) {
 	t.Skip() // travil fails.