Calculate TPI for irregular DEM

Question

When I use SAGA's Tool Topographic Position Index (TPI) or Tool TPI Based Landform Classification (in QGIS or SAGA itself) on a rectangular DEM the results are as expected but when I try them on a DEM with irregular borders I get the following result:

Is it possible to calculate TPI on this kind of DEM or do I need to subset it on multiple rectangular tiles?

Answer 1

I found another Question which gave me a hint on what the problem could be. SAGA recognize No Data values as -99999. Other values get processed as regular. I solved my problem reclassifying No Data values (which were being read as -3.4e+38) as -99999.