diff --git a/core/blockchain.go b/core/blockchain.go
index c852926ef4..feb9fb9426 100644
--- a/core/blockchain.go
+++ b/core/blockchain.go
@@ -979,20 +979,26 @@ func (bc *BlockChain) writeBlockWithState(block *types.Block, receipts []*types.
 				triedb.Cap(limit - ethdb.IdealBatchSize)
 			}
 			// Find the next state trie we need to commit
-			header := bc.GetHeaderByNumber(current - triesInMemory)
-			chosen := header.Number.Uint64()
+			chosen := current - triesInMemory
 
 			// If we exceeded out time allowance, flush an entire trie to disk
 			if bc.gcproc > bc.cacheConfig.TrieTimeLimit {
-				// If we're exceeding limits but haven't reached a large enough memory gap,
-				// warn the user that the system is becoming unstable.
-				if chosen < lastWrite+triesInMemory && bc.gcproc >= 2*bc.cacheConfig.TrieTimeLimit {
-					log.Info("State in memory for too long, committing", "time", bc.gcproc, "allowance", bc.cacheConfig.TrieTimeLimit, "optimum", float64(chosen-lastWrite)/triesInMemory)
+				// If the header is missing (canonical chain behind), we're reorging a low
+				// diff sidechain. Suspend committing until this operation is completed.
+				header := bc.GetHeaderByNumber(chosen)
+				if header == nil {
+					log.Warn("Reorg in progress, trie commit postponed", "number", chosen)
+				} else {
+					// If we're exceeding limits but haven't reached a large enough memory gap,
+					// warn the user that the system is becoming unstable.
+					if chosen < lastWrite+triesInMemory && bc.gcproc >= 2*bc.cacheConfig.TrieTimeLimit {
+						log.Info("State in memory for too long, committing", "time", bc.gcproc, "allowance", bc.cacheConfig.TrieTimeLimit, "optimum", float64(chosen-lastWrite)/triesInMemory)
+					}
+					// Flush an entire trie and restart the counters
+					triedb.Commit(header.Root, true)
+					lastWrite = chosen
+					bc.gcproc = 0
 				}
-				// Flush an entire trie and restart the counters
-				triedb.Commit(header.Root, true)
-				lastWrite = chosen
-				bc.gcproc = 0
 			}
 			// Garbage collect anything below our required write retention
 			for !bc.triegc.Empty() {
@@ -1324,7 +1330,7 @@ func (bc *BlockChain) insertSidechain(block *types.Block, it *insertIterator) (i
 			if err := bc.WriteBlockWithoutState(block, externTd); err != nil {
 				return it.index, nil, nil, err
 			}
-			log.Debug("Inserted sidechain block", "number", block.Number(), "hash", block.Hash(),
+			log.Debug("Injected sidechain block", "number", block.Number(), "hash", block.Hash(),
 				"diff", block.Difficulty(), "elapsed", common.PrettyDuration(time.Since(start)),
 				"txs", len(block.Transactions()), "gas", block.GasUsed(), "uncles", len(block.Uncles()),
 				"root", block.Root())
diff --git a/core/blockchain_test.go b/core/blockchain_test.go
index cfc44bf794..fa68c2894a 100644
--- a/core/blockchain_test.go
+++ b/core/blockchain_test.go
@@ -1483,3 +1483,58 @@ func BenchmarkBlockChain_1x1000Executions(b *testing.B) {
 
 	benchmarkLargeNumberOfValueToNonexisting(b, numTxs, numBlocks, recipientFn, dataFn)
 }
+
+// Tests that importing a very large side fork, which is larger than the canon chain,
+// but where the difficulty per block is kept low: this means that it will not
+// overtake the 'canon' chain until after it's passed canon by about 200 blocks.
+//
+// Details at:
+//  - https://github.com/ethereum/go-ethereum/issues/18977
+//  - https://github.com/ethereum/go-ethereum/pull/18988
+func TestLowDiffLongChain(t *testing.T) {
+	// Generate a canonical chain to act as the main dataset
+	engine := ethash.NewFaker()
+	db := ethdb.NewMemDatabase()
+	genesis := new(Genesis).MustCommit(db)
+
+	// We must use a pretty long chain to ensure that the fork doesn't overtake us
+	// until after at least 128 blocks post tip
+	blocks, _ := GenerateChain(params.TestChainConfig, genesis, engine, db, 6*triesInMemory, func(i int, b *BlockGen) {
+		b.SetCoinbase(common.Address{1})
+		b.OffsetTime(-9)
+	})
+
+	// Import the canonical chain
+	diskdb := ethdb.NewMemDatabase()
+	new(Genesis).MustCommit(diskdb)
+
+	chain, err := NewBlockChain(diskdb, nil, params.TestChainConfig, engine, vm.Config{}, nil)
+	if err != nil {
+		t.Fatalf("failed to create tester chain: %v", err)
+	}
+	if n, err := chain.InsertChain(blocks); err != nil {
+		t.Fatalf("block %d: failed to insert into chain: %v", n, err)
+	}
+	// Generate fork chain, starting from an early block
+	parent := blocks[10]
+	fork, _ := GenerateChain(params.TestChainConfig, parent, engine, db, 8*triesInMemory, func(i int, b *BlockGen) {
+		b.SetCoinbase(common.Address{2})
+	})
+
+	// And now import the fork
+	if i, err := chain.InsertChain(fork); err != nil {
+		t.Fatalf("block %d: failed to insert into chain: %v", i, err)
+	}
+	head := chain.CurrentBlock()
+	if got := fork[len(fork)-1].Hash(); got != head.Hash() {
+		t.Fatalf("head wrong, expected %x got %x", head.Hash(), got)
+	}
+	// Sanity check that all the canonical numbers are present
+	header := chain.CurrentHeader()
+	for number := head.NumberU64(); number > 0; number-- {
+		if hash := chain.GetHeaderByNumber(number).Hash(); hash != header.Hash() {
+			t.Fatalf("header %d: canonical hash mismatch: have %x, want %x", number, hash, header.Hash())
+		}
+		header = chain.GetHeader(header.ParentHash, number-1)
+	}
+}
diff --git a/core/chain_makers.go b/core/chain_makers.go
index e3a5537a40..0b5a3d1843 100644
--- a/core/chain_makers.go
+++ b/core/chain_makers.go
@@ -149,7 +149,7 @@ func (b *BlockGen) PrevBlock(index int) *types.Block {
 // associated difficulty. It's useful to test scenarios where forking is not
 // tied to chain length directly.
 func (b *BlockGen) OffsetTime(seconds int64) {
-	b.header.Time.Add(b.header.Time, new(big.Int).SetInt64(seconds))
+	b.header.Time.Add(b.header.Time, big.NewInt(seconds))
 	if b.header.Time.Cmp(b.parent.Header().Time) <= 0 {
 		panic("block time out of range")
 	}