diff --git a/src/main/java/de/neemann/digital/core/io/MIDIHelper.java b/src/main/java/de/neemann/digital/core/io/MIDIHelper.java
index 291b4c63f..d77a8633a 100644
--- a/src/main/java/de/neemann/digital/core/io/MIDIHelper.java
+++ b/src/main/java/de/neemann/digital/core/io/MIDIHelper.java
@@ -70,9 +70,9 @@ public final class MIDIHelper {
     }
 
     private void close() {
-        if (!isOpen) {
+        if (isOpen) {
             synthesizer.close();
-            synthesizer = null;
+            isOpen = false;
         }
     }