diff --git a/deidentifier-pipeline/src/main/java/org/ratschlab/deidentifier/workflows/WriteDocsToDatabase.java b/deidentifier-pipeline/src/main/java/org/ratschlab/deidentifier/workflows/WriteDocsToDatabase.java
index ede8271..208bc19 100644
--- a/deidentifier-pipeline/src/main/java/org/ratschlab/deidentifier/workflows/WriteDocsToDatabase.java
+++ b/deidentifier-pipeline/src/main/java/org/ratschlab/deidentifier/workflows/WriteDocsToDatabase.java
@@ -51,7 +51,7 @@ public void doneHook() {
         }
     }
 
-    private void flushQueue() throws SQLException {
+    private synchronized void flushQueue() throws SQLException {
         List<Pair<String, Map<Object, Object>>> lst = new ArrayList<>();
 
         while (!writeQueue.isEmpty()) {