C.O.
Personally? I would call - as people's ideas and/or perceptions of formal attire are different...to me - although it doesn't say "black tie" - I would dress like it was a black tie event...
Also depends upon where it is - if it's at a 5 star location and in the evening - then they probably mean gowns and VERY nice suit or tux!!
Sorry I can't be more help!!