network analyst - GRASS v.net.iso tool produces empty output - QGIS 3.2.2

I want to split a network shapefile by cost isolines.

I have two layers

• a point layer (with 297 records)


• a road network layer

Every time I run the GRASS v.net.iso module in QGIS 3.2.2 on the road network layer, the output will be an empty shapefile with an empty attribute table.

Can anyone tell me how to use the v.net.iso tool correctly?

I couldn't find a tutorial online for the v.net.iso tool.