diff --git a/backend/src/api/websocket-handler.ts b/backend/src/api/websocket-handler.ts
index ed3d007c4..cea0ce781 100644
--- a/backend/src/api/websocket-handler.ts
+++ b/backend/src/api/websocket-handler.ts
@@ -371,8 +371,8 @@ class WebsocketHandler {
           }
 
           if (parsedMessage && parsedMessage['track-mempool-block'] !== undefined) {
-            if (Number.isInteger(parsedMessage['track-mempool-block']) && parsedMessage['track-mempool-block'] >= 0) {
-              const index = parsedMessage['track-mempool-block'];
+            const index = parsedMessage['track-mempool-block'];
+            if (Number.isInteger(index) && index >= 0 && index < config.MEMPOOL.MEMPOOL_BLOCKS_AMOUNT) {
               client['track-mempool-block'] = index;
               const mBlocksWithTransactions = mempoolBlocks.getMempoolBlocksWithTransactions();
               response['projected-block-transactions'] = JSON.stringify({