plus operator not working in custom forensic logging format

I've tried configuring kea-dhcp4 and kea-dhcp6 with custom formats with pluses. I get the following error:

ERROR LEGAL_LOG_LOAD_ERROR LEGAL LOGGING DISABLED! An error occurred loading the library: <string>: Invalid character: +
ERROR HOOKS_LOAD_ERROR 'load' function in hook library /opt/kea/lib/kea/hooks/libdhcp_legal_log.so returned error 1

    "request-parser-format": "'kea ' + 'is awesome'"