diff --git a/src/org/pEp/jniadapter/AbstractEngine.java b/src/org/pEp/jniadapter/AbstractEngine.java
index 57ef25e..5754886 100644
--- a/src/org/pEp/jniadapter/AbstractEngine.java
+++ b/src/org/pEp/jniadapter/AbstractEngine.java
@@ -30,7 +30,9 @@ abstract class AbstractEngine implements AutoCloseable {
     }
 
     final public void close() {
-        release();
+        synchronized (AbstractEngine.class){
+            release();
+        }
     }
 
     private long keyserverThread;