How to sum annotation values ​by groupby in Django

In Django, an error occurs when calculating (Sum) with the value obtained by annotate rather than as a unique field of the model. Is it possible to calculate with only unique fields? I want to find Sum by groupby 'enroll_injection' value into 'student__research' .

enroll_injection = test.filter(Q(student__is_deleted=0))\
                       .values('student')\
                       .annotate(injection=Max('injection_date'),
                                 enroll=Max('enroll_date'))\
                       .values('student__research')\
                       .annotate(enroll_injection=Cast(F('enroll_date'), DateField()) - Cast(F('injection_date'), DateField()))\
                       .values('assignment__research__research_name', 'enroll_injection')\
                       .order_by('assignment__research__research_name')

Error: Cannot compute Sum('<CombinedExpression: Cast(F(enroll_date)) - Cast(F(injection_date))>'): '<CombinedExpression: Cast(F(enroll_date)) - Cast(F(injection_date))>' is an aggregate

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