If you look at the build-pysnmp-mib script , it uses smidump. So your smidump program is probably not parsing your FORTINET-FORTIGATE-MIB.mib file correctly because you don't have your smidump environment setup correctly to resolve the mib dependencys that FORTINET-FORTIGATE-MIB.mib refers to.Ergo, it pipes through "empty input" to the libsmi2pysnmp program which is what turns the smidump output into python modules for the SNMP Modular Input to load. In my environment , there are entrys in /etc/smi.conf for the directorys where mibs live that will be resolved by smidump.
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