diff --git a/src/main/java/de/neemann/digital/hdl/model2/HDLNet.java b/src/main/java/de/neemann/digital/hdl/model2/HDLNet.java
index 0290aa473..698664ac9 100644
--- a/src/main/java/de/neemann/digital/hdl/model2/HDLNet.java
+++ b/src/main/java/de/neemann/digital/hdl/model2/HDLNet.java
@@ -203,6 +203,8 @@ public class HDLNet implements Printable, HasName {
      * @return true if this is a clock net
      */
     public boolean isClock() {
+        if (output == null)
+            return false;
         return output.isClock();
     }