diff --git a/src/rpc/blockchain.h b/src/rpc/blockchain.h
--- a/src/rpc/blockchain.h
+++ b/src/rpc/blockchain.h
@@ -5,6 +5,8 @@
 #ifndef BITCOIN_RPC_BLOCKCHAIN_H
 #define BITCOIN_RPC_BLOCKCHAIN_H
 
+#include <sync.h>
+
 #include <univalue.h>
 
 class CBlock;
@@ -13,6 +15,8 @@
 class CTxMemPool;
 class JSONRPCRequest;
 
+extern RecursiveMutex cs_main;
+
 UniValue getblockchaininfo(const Config &config, const JSONRPCRequest &request);
 
 /**
@@ -28,7 +32,8 @@
 
 /** Block description to JSON */
 UniValue blockToJSON(const CBlock &block, const CBlockIndex *tip,
-                     const CBlockIndex *blockindex, bool txDetails = false);
+                     const CBlockIndex *blockindex, bool txDetails = false)
+    LOCKS_EXCLUDED(cs_main);
 
 /** Mempool information to JSON */
 UniValue MempoolInfoToJSON(const CTxMemPool &pool);
@@ -38,6 +43,7 @@
 
 /** Block header to JSON */
 UniValue blockheaderToJSON(const CBlockIndex *tip,
-                           const CBlockIndex *blockindex);
+                           const CBlockIndex *blockindex)
+    LOCKS_EXCLUDED(cs_main);
 
 #endif // BITCOIN_RPC_BLOCKCHAIN_H
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -86,6 +86,9 @@
                            const CBlockIndex *blockindex) {
     // Serialize passed information without accessing chain state of the active
     // chain!
+    // For performance reasons
+    AssertLockNotHeld(cs_main);
+
     UniValue result(UniValue::VOBJ);
     result.pushKV("hash", blockindex->GetBlockHash().GetHex());
     const CBlockIndex *pnext;
@@ -117,6 +120,9 @@
                      const CBlockIndex *blockindex, bool txDetails) {
     // Serialize passed information without accessing chain state of the active
     // chain!
+    // For performance reasons
+    AssertLockNotHeld(cs_main);
+
     UniValue result(UniValue::VOBJ);
     result.pushKV("hash", blockindex->GetBlockHash().GetHex());
     const CBlockIndex *pnext;