fixup

yt/fields/field_info_container.py

@@ -473,11 +473,19 @@ def alias(
         if units is None:
             # We default to CGS here, but in principle, this can be pluggable
             # as well.
-            u = Unit(self[original_name].units, registry=self.ds.unit_registry)
-            if u.dimensions is not dimensionless:
-                units = str(self.ds.unit_system[u.dimensions])
+
+            # self[original_name].units may be set to `None` at this point
+            # to signal that units should be autoset later
+            oru = self[original_name].units
+            if oru is None:
+                units = None
             else:
-                units = self[original_name].units
+                u = Unit(oru, registry=self.ds.unit_registry)
+                if u.dimensions is not dimensionless:
+                    units = str(self.ds.unit_system[u.dimensions])
+                else:
+                    units = oru
+
         self.field_aliases[alias_name] = original_name
         function = TranslationFunc(original_name)
         if deprecate is not None: