[R] Raster extract by polygon generating NAs

[Joao Carreiras]

Greetings!

I'm using the extract command (raster package) with a raster layer (x)
and a spatial polygons dataframe (y). I'm using it to extract the sum
of raster values by each spatial polygon. However, I'm getting NAs as
a result for some polygon IDs, which I know isn't true because that
doesn't happen in ArcGIS.

Does anyone experienced the same issue?

Thanks!
Joao