ERROR Virtual Start Execute failed: Output objects in virtual input node have not been set


I'm experiencing an error with the parallel loop nodes. Sometimes (I cannot really understand when and why it happens) I obtain the following error:

ERROR Virtual Start 0:1943:1633:1014:1026 Execute failed: Output objects in virtual input node have not been set

Nevertheless it seems that the output is correct and complete. Do I have to worry about this error message or can I ignore it?

Any opinion is appreciated,


Honestly, I'm slightly concerned because that doesn't sound right. Do you have a workflow that we can use to reproduce this?

I checked the code. The node would fail if this error is printed. So I wonder why your parallel loop completes (does it show the green traffic light and the virtual nodes disappear again?).

Hello Wiswedel, thank you for your reply.

Now it seems I cannot reproduce this problem but I can tell you it was linked to fact I tried to execute this internal parallelized loop inside a partially executed external loop. If I remember correctly the problem disappeared after I manually deleted the virtual metanode that is automatically created when the parallel loop is executed and then I restarted the external loop from the beginning. I hope this can help if other people run into this problem.

I will write here more details if the problem occurs again.

Thank you for your help.



OK, thanks. So that was caused by some parallel looper that was half-way executed and then started without a proper reset before? Yes, that should be fixed and we should better warn if these semi-executed loops are resumed (in fact we do for partially executed loops that get loaded from disk).

... that's one of the reasons why the parallel loop construct is still in labs. You need to clean-up yourself if it fails.


6 years later and this issue still happening for me, except I did reset the Parallel Chunk Loop before executing. Any ideas what else causes this?

Dear All,
this was a helpful chat. I had the same issue on the KNIME server. After I deleted the virtual node prior to uploading the workflow to the server, the error message disappeared.