Refs #26565 -- Errored nicely when using Prefetch with a values() queryset.

Thanks Maxime Lorant for the report and Anssi for the suggestion.

django/db/models/query.py

@@ -1331,6 +1331,8 @@ class Prefetch(object):
         self.prefetch_through = lookup
         # `prefetch_to` is the path to the attribute that stores the result.
         self.prefetch_to = lookup
+        if queryset is not None and queryset._iterable_class is not ModelIterable:
+            raise ValueError('Prefetch querysets cannot use values().')
         if to_attr:
             self.prefetch_to = LOOKUP_SEP.join(lookup.split(LOOKUP_SEP)[:-1] + [to_attr])