Excel help – Data validation tables and VLOOKUP

I was wondering why my VLOOKUP formula is not working, I am using data validation to select the values I have a table like this,

Smooth concrete or asphalt 1.000
Stone blocks pitched or mortared 0.950
Turf 0.880
Stepped 0.850
One layer of rocks on impermeable base 0.700
Two layers of rock armour 0.550

whenever I select ‘stepped’ it comes up with a value of 1, and whenever I select ‘One layer of rocks on impermeable base’ it comes up with N/A

The forumla is

=IF(C18<>“”,VLOOKUP(C18,F64:G69,2),””)
Where c18 is the data validation drop down menu

Any help will be gratefully recieved