[QGIS-Developer] what does geos geoprocessing error difference failed mean?

[Shane Carey]

Hi,

I'm running a union process in qgis 3.4 and I keep getting the following
error:
*geos geoprocessing error difference failed*
What does this error mean?
Thanks
Le gach dea ghui,
*Shane Carey*
*GIS and Data Solutions Consultant*
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