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coordinate system - $area gives NULL for some polygons in QGIS's Field Calculator - Geographic Information Systems Stack Exchange
![Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub](https://user-images.githubusercontent.com/87969978/133457170-a834a12d-fe75-4744-9586-ded78934039c.jpg)
Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub
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qgis - NULL values when using overlay_within function in Field Calculator - Geographic Information Systems Stack Exchange
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New values created with QGIS field calculator become null upon saving - Geographic Information Systems Stack Exchange
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openstreetmap - Why blank values in attribute table don't work as NULL in QGIS? - Geographic Information Systems Stack Exchange
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New values created with QGIS field calculator become null upon saving - Geographic Information Systems Stack Exchange
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Checking if attribute name exists using QGIS Field Calculator - Geographic Information Systems Stack Exchange
![Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub](https://user-images.githubusercontent.com/16478283/108544391-0b768900-72ef-11eb-887a-3b73aa12a7f9.png)
Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub
![Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub](https://user-images.githubusercontent.com/52182284/132603015-a377d308-75fd-4fc3-951d-af1173ed3bd4.png)
Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub
![Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub Custom Function returns NULL value when run in Field Calculator (Preview works fine) · Issue #41695 · qgis/QGIS · GitHub](https://user-images.githubusercontent.com/7983394/108952929-9eeee780-766a-11eb-9ce5-219f035cdc53.gif)