If you think PCRaster did something incorrect please file a bug report. Any error messages containing the phrases: "assertion failed" or "programming error" are always related to errors in the software we should fix. Submit bug reports to the issues page on the PCRaster GitHub repository containing:
The problem description in as much detail as possible.
The PCRaster version and the Python version used
A zip or archive file with all data and scripts so we can reproduce the bug. If you have a large model (long script and/or much data) try to make a simpler model that still gives the same error.
Try sending an attachment of your screen showing possible error messages:
Press PrntScrn (PrintScreen) on your Keyboard
Open Paint: Start->All Programs->Accessories->Paint
Copy Screen in untitled: Paint Menu->Edit->Paste
Save graphics File: File->Save->(type .PNG is preferred)
Attach the saved file to your bug report
In case you can provide a code fix yourself please consider to submit a pull request.