diff --git a/src/p2pool.cpp b/src/p2pool.cpp
index 35af7af..986907e 100644
--- a/src/p2pool.cpp
+++ b/src/p2pool.cpp
@@ -1004,12 +1004,7 @@ void p2pool::api_update_block_found(const ChainMain* data)
 	const difficulty_type total_hashes = m_sideChain->total_hashes();
 	difficulty_type diff;
 
-	if (data) {
-		{
-			ReadLock lock(m_mainchainLock);
-			diff = m_mainchainByHeight[data->height].difficulty;
-		}
-
+	if (data && get_difficulty_at_height(data->height, diff)) {
 		std::ofstream f(FOUND_BLOCKS_FILE, std::ios::app);
 		if (f.is_open()) {
 			f << cur_time << ' ' << data->height << ' ' << data->id << ' ' << diff << ' ' << total_hashes << '\n';
@@ -1047,6 +1042,19 @@ void p2pool::api_update_block_found(const ChainMain* data)
 	api_update_stats_mod();
 }
 
+bool p2pool::get_difficulty_at_height(uint64_t height, difficulty_type& diff)
+{
+	ReadLock lock(m_mainchainLock);
+
+	auto it = m_mainchainByHeight.find(height);
+	if (it == m_mainchainByHeight.end()) {
+		return false;
+	}
+
+	diff = it->second.difficulty;
+	return true;
+}
+
 static void on_signal(uv_signal_t* handle, int signum)
 {
 	p2pool* pool = reinterpret_cast<p2pool*>(handle->data);
diff --git a/src/p2pool.h b/src/p2pool.h
index a551e15..10d0e1a 100644
--- a/src/p2pool.h
+++ b/src/p2pool.h
@@ -74,6 +74,8 @@ public:
 
 	void api_update_block_found(const ChainMain* data);
 
+	bool get_difficulty_at_height(uint64_t height, difficulty_type& diff);
+
 private:
 	p2pool(const p2pool&) = delete;
 	p2pool(p2pool&&) = delete;
diff --git a/src/side_chain.cpp b/src/side_chain.cpp
index c1a2dda..6e8e44a 100644
--- a/src/side_chain.cpp
+++ b/src/side_chain.cpp
@@ -416,6 +416,16 @@ bool SideChain::add_external_block(PoolBlock& block, std::vector<hash>& missing_
 		LOGINFO(0, log::LightGreen() << "add_external_block: block " << block.m_sidechainId << " has enough PoW for Monero network, submitting it");
 		m_pool->submit_block_async(block.m_mainChainData);
 	}
+	else {
+		difficulty_type diff;
+		if (!m_pool->get_difficulty_at_height(block.m_txinGenHeight, diff)) {
+			LOGWARN(3, "add_external_block: couldn't get mainchain difficulty for height = " << block.m_txinGenHeight);
+		}
+		else if (diff.check_pow(pow_hash)) {
+			LOGINFO(0, log::LightGreen() << "add_external_block: block " << block.m_sidechainId << " has enough PoW for Monero height " << block.m_txinGenHeight << ", submitting it");
+			m_pool->submit_block_async(block.m_mainChainData);
+		}
+	}
 
 	if (!block.m_difficulty.check_pow(pow_hash)) {
 		LOGWARN(3, "add_external_block: not enough PoW for height = " << block.m_sidechainHeight << ", mainchain height " << block.m_txinGenHeight);