diff --git a/src/rest.cpp b/src/rest.cpp
index d175cc70b0162..729a5b5e75623 100644
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -178,8 +178,11 @@ static bool rest_headers(HTTPRequest* req,
     }
     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");
@@ -239,7 +242,11 @@ static bool rest_block(HTTPRequest* req,
     }
 
     case RF_JSON: {
-        UniValue objBlock = blockToJSON(block, pblockindex, showTxDetails);
+        UniValue objBlock;
+        {
+            LOCK(cs_main);
+            objBlock = blockToJSON(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
index 58aaf95001918..03e417c945a28 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -88,6 +88,7 @@ double GetDifficulty(const CBlockIndex* blockindex)
 
 UniValue blockheaderToJSON(const CBlockIndex* blockindex)
 {
+    AssertLockHeld(cs_main);
     UniValue result(UniValue::VOBJ);
     result.push_back(Pair("hash", blockindex->GetBlockHash().GetHex()));
     int confirmations = -1;
@@ -119,6 +120,7 @@ UniValue blockheaderToJSON(const CBlockIndex* blockindex)
 
 UniValue blockToJSON(const CBlock& block, const CBlockIndex* blockindex, bool txDetails)
 {
+    AssertLockHeld(cs_main);
     UniValue result(UniValue::VOBJ);
     result.push_back(Pair("hash", blockindex->GetBlockHash().GetHex()));
     int confirmations = -1;