diff --git a/particle.py b/particle.py
index ce562b3..3a527e8 100644
--- a/particle.py
+++ b/particle.py
@@ -154,6 +154,8 @@ class Particles:
                 idxlo = np.array(fld.nearest_gridpoint(pos-rp,lower=True))
                 sfdim = np.ceil(2*rp/fld.spacing+2).astype('int')
                 sf = fld.extract_subfield(idxlo,sfdim,deep=False,strict_bounds=False)
+                if sf is None:
+                    continue
                 xsf = sf.x().reshape((-1,1,1))
                 ysf = sf.y().reshape((1,-1,1))
                 zsf = sf.z().reshape((1,1,-1))