error problem with 1582 Incorrect parameter count in the call to native function 'ISNULL'

hello everyone
i have 2 tables one (visits) and other one (patients) with relationship by mysql (patients) is with PK as patients.pid and visits.pid as index
and i wanted to use code in my vb.net app to get the last datetime record to check the patient if still inside the hospital admitted or discharge
so far i have this code with help of you guys

SELECT a.pid,MAX(ISNULL(b.sdat,'1901-01-01')),MAX(ISNULL(b.edat,'1901-01-01')) from patients a left join visits b on a.pid=b.pid Where ddatediff(now(),b.edat) <=365 group by a.pid

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but when i execute in sql builder using php myadmin  or another app i get error
(1582 - Incorrect parameter count in the call to native function 'ISNULL')
i tried to find out the error cause but no luck until now