DAX Measure Summarize error

Hi Experts

I cannot see where my error is on the following Summarizse, SumX DAX measure

VAR _a = SUMX(
            SUMMARIZE(shc_practice_list,shc_practice_list[parent_external_hco_code],shc_practice_list[practice_name], "List", SUM(shc_practice_list[list_size])),[List])