Zabbix assumes that when you use a dynamic index, it will be an integer value. In some cases, enterprise MIBs contain OIDs with variable length indexes. Zabbix treats the last byte of the variable length index as the internal index. Here is the section of code that looks like it might be causing the problem

In src/zabbix_server/poller/checks_snmp.c

— around Line 433 —
if (0 == strcmp(value, strval))

{ *idx = vars->name[vars->name_length - 1]; zabbix_log(LOG_LEVEL_DEBUG, "FOUND: Index is %d", *idx); ret = SUCCEED; running = 0; }

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It seems like the various index caching mechanisms will also need to be updated to support variable length indexes.