Count Function in an Update Query

How do I update a field in 1 table with the count of matching records in a 
2nd table? I keep getting error 3122 when I attempt this. 

My example is

UPDATE Table1 INNER JOIN Table2 ON Table1.Field2 = Table2.Field2
SET Table1.Field1 = COUNT(Table2.Field2)
GROUP BY Table2.Field2 ;