Selecting features with NULL attributes in QGIS?

How do I identify which features have NULL attributes in the attribute table of a shapefile?

When i try [field]= 'NULL' it selects the features with value = 0 (see picture)

Answer

Have you tried replacing the "=" with "is"? I don't have QGIS in front of me, but that is pretty common database select syntax for working with NULL values.

pedonal is NULL