unirecfilter: Unable to filter fields containing number

Unirec filter is unable to filter L4 fields from ipfixprobe. Using any fields, which contains number will results in filter SYNTAX ERROR. 

Example:
field:  "L4_TCP_SYN_SIZE"
Error message: "[URFilter] Syntax error in filter: L4_TCP_SYN_SIZE != 0"
Using "L_TCP_SYN_SIZE" or "TCP_SYN_SIZE" will results in warning "[field] is not present in input format"