FDB table not working on fortinet switches

as it says FDB table discovery doesn’t work on fortinet switches. (I believe all of our switches are > version 7.0.x)
it mentions a log, for more details, but not which one.
version is 22.4.0-21-g738ed538e, validate.php is happy.
I also tried using the latest MIBS from fortinet but no change.
from the end of the capture debug information->Discovery for the switch:

SNMP['/usr/bin/snmpbulkwalk' '-v3' '-l' 'authPriv' '-n' "" '-a' 'XXX' '-A' 'PASSWORD' '-u' 'USER' '-x' 'XXX' '-X' 'PASSWORD' '-X' '-m' 'Q-BRIDGE-MIB' '-M' '/opt/librenms/mibs:/opt/librenms/mibs/fortinet' 'udp:HOSTNAME:161' 'dot1qVlanFdbId']

dot1qVlanFdbId[1] = 0
..snip..
dot1qVlanFdbId[73] = 0  

Error discovering fdb-table module for x.x.x.x. TypeError: Cannot access offset of type string on string in /opt/librenms/includes/discovery/fdb-table/bridge.inc.php:62
Stack trace:
#0 /opt/librenms/includes/discovery/fdb-table.inc.php(29): include()
#1 /opt/librenms/includes/discovery/functions.inc.php(157): include('...')
#2 /opt/librenms/discovery.php(118): discover_device()
#3 {main}  

SQL[insert into `eventlog` (`device_id`, `reference`, `type`, `datetime`, `severity`, `message`, `username`) values (?, ?, ?, ?, ?, ?, ?) [185,null,"discovery","2022-05-09 14:39:39",5,"Error discovering fdb-table module. Check log file for more details.",""] 1.14ms] 

>> Runtime for discovery module 'fdb-table': 6.9760 seconds with 1054200 bytes
>> SNMP: [0/0.00s] MySQL: [56/0.18s] RRD: [0/0.00s]  

#### Unload disco module fdb-table ####