diff --git a/src/main/java/com/iota/iri/service/API.java b/src/main/java/com/iota/iri/service/API.java
index bd965dcfa7..0b5fb97caa 100644
--- a/src/main/java/com/iota/iri/service/API.java
+++ b/src/main/java/com/iota/iri/service/API.java
@@ -572,7 +572,7 @@ private static void incEllapsedTimeGetTxToApprove(long ellapsedTime) {
       * @return {@link com.iota.iri.service.dto.GetTransactionsToApproveResponse}
       **/
     @Document(name="getTransactionsToApprove")
-    private AbstractResponse getTransactionsToApproveStatement(int depth, Optional<Hash> reference) {
+    private synchronized AbstractResponse getTransactionsToApproveStatement(int depth, Optional<Hash> reference) {
         if (depth < 0 || depth > configuration.getMaxDepth()) {
             return ErrorResponse.create("Invalid depth input");
         }