VLOOKUP does not return a value

I am using Excel 2003, and doing some engineering work.  I have set up a table where the lookup value, on the far left of the table, is a value expressed in inches.  The values are 1.5", 2", 3", 4", 6" and 8".  VLOOKUP will retrun correct values for all of lookup values other than for 1.5".  At any cell where the lookup value is 1.5", I get "#N/A".

 

Any idea as to what is causing this to ahppen and what I can do to correct it?