diff --git a/field.py b/field.py
index 084aa5e..be8ac34 100644
--- a/field.py
+++ b/field.py
@@ -150,7 +150,7 @@ class Field3d:
 
     def insert_subfield(self,subfield):
         assert all([abs(subfield.spacing[ii]-self.spacing[ii])<self.eps_collapse 
-                        for ii in range(3)]), "spacing differs."
+                        for ii in range(3)]), "spacing differs. Got {}, have {}".format(subfield.spacing,self.spacing)
         assert all([self.distance_to_nearest_gridpoint(subfield.origin[ii],axis=ii)<self.eps_collapse 
                         for ii in range(3)]), "subfield has shifted origin."
         assert all(self.is_within_bounds(subfield.origin,axis=None)), "subfield origin is out-of-bounds."