diff --git a/yt/data_objects/selection_objects/data_selection_objects.py b/yt/data_objects/selection_objects/data_selection_objects.py
index a4cc4fe612e..0a4081882ea 100644
--- a/yt/data_objects/selection_objects/data_selection_objects.py
+++ b/yt/data_objects/selection_objects/data_selection_objects.py
@@ -262,6 +262,8 @@ def _generate_fields(self, fields_to_generate):
                         fi.units = sunits
                         fi.dimensions = dimensions
                         self.field_data[field] = self.ds.arr(fd, units)
+                    if fi.output_units is None:
+                        fi.output_units = fi.units
 
                     try:
                         fd.convert_to_units(fi.units)