diff --git a/src/rest.cpp b/src/rest.cpp
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -198,8 +198,11 @@
         }
         case RF_JSON: {
             UniValue jsonHeaders(UniValue::VARR);
-            for (const CBlockIndex *pindex : headers) {
-                jsonHeaders.push_back(blockheaderToJSON(pindex));
+            {
+                LOCK(cs_main);
+                for (const CBlockIndex *pindex : headers) {
+                    jsonHeaders.push_back(blockheaderToJSON(pindex));
+                }
             }
             std::string strJSON = jsonHeaders.write() + "\n";
             req->WriteHeader("Content-Type", "application/json");
@@ -271,8 +274,12 @@
         }
 
         case RF_JSON: {
-            UniValue objBlock =
-                blockToJSON(config, block, pblockindex, showTxDetails);
+            UniValue objBlock;
+            {
+                LOCK(cs_main);
+                objBlock =
+                    blockToJSON(config, block, pblockindex, showTxDetails);
+            }
             std::string strJSON = objBlock.write() + "\n";
             req->WriteHeader("Content-Type", "application/json");
             req->WriteReply(HTTP_OK, strJSON);
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -68,6 +68,7 @@
 }
 
 UniValue blockheaderToJSON(const CBlockIndex *blockindex) {
+    AssertLockHeld(cs_main);
     UniValue result(UniValue::VOBJ);
     result.pushKV("hash", blockindex->GetBlockHash().GetHex());
     int confirmations = -1;
@@ -100,6 +101,7 @@
 
 UniValue blockToJSON(const Config &config, const CBlock &block,
                      const CBlockIndex *blockindex, bool txDetails) {
+    AssertLockHeld(cs_main);
     UniValue result(UniValue::VOBJ);
     result.pushKV("hash", blockindex->GetBlockHash().GetHex());
     int confirmations = -1;